The study investigated the effects of season and corticosterone treatment on populations of Plasmodium relictum in house sparrows (Passer domesticus). Birds from an experimentally infected field population were returned to the laboratory on 22 January, 4 March, and 14 April, where half received 250 /,g corticosterone in 0.05 ml peanut oil per day and half received 0.05 ml oil per day for 10 days. Corticosterone treatment was consistently accompanied by a higher proportion of birds with patent infections, a higher proportion of blood films with demonstrable parasites, and a higher proportion of infected erythrocytes than was observed in the oil control group. Parasites were more prevalent in the blood of both experimental and control birds as season advanced from winter to spring. The relapse phenomenon in malaria remains poorly understood. While such diverse treatments as fatigue, chilling, sunburn, anoxia, surgical procedures, alcohol, and epinephrine injections have reportedly caused latent infections to become overt, the mechanism involved in malarial relapse has not been elucidated (Coatney and Cooper, 1948; Farmer, 1962). Since many of these treatments are common stressing agents used in adrenal research, mediation by an adrenal corticoid might be suspected. Evidence has indicated that spring relapses occur in avian plasmodial infections (Box, 1966; Applegate, 1968) and that avian adrenal cortical activity has an annual rhythm, likewise with a maximum in the spring of the year (John, 1966; Raitt, 1968). The present study investigates the effects of season and corticosterone treatment on populations of Plasmodium relictum in house sparrows (Passer domesticus). MATERIALS AND METHODS The experimental host used in this study was the English sparrow (Passer domesticus (Linnaeus)). The parasite used was a form of PlasReceived for publication 23 July 1969. * From a doctoral dissertation in zoology submitted to the graduate faculty of The Pennsylvania State University. Bureau of Medicine and Surgery, Navy Department, Research Task MR005 05 0004. This investigation was supported in part by Public Health Service Fellowship number 5-F01-GM-37661-02 from the General Medical Services Division. The opinions and assertions contained herein are those of the author and are not to be construed as official or reflecting the views of the Navy Department or the naval service at large. modium relictum (Grassi and Feletti, 1891) known to relapse and to infect English sparrows. All birds were infected by subinoculation in the jugular vein using a 1-ml heparin rinsed tuberculin syringe fitted with a 27-gauge needle. Thin blood films were prepared from blood collected by clipping the end of a toenail. The films were fixed in methyl alcohol, stained in Giemsa's stain, and examined under oil immersion optics (970 X ). Each slide was studied 5 min. Approximately 40,000 cells were scanned in each film by this protocol. Birds were kept in 12 by 10 by 16-inch cages in the laboratory and fed canary and finch seed daily. The room in which the birds were kept was maintained at 24 C and light was controlled at a constant 12-hr cycle from 6:00 AM to 6:00 PM. Corticosterone used in this study was obtained from National Biochemical Corporation. Twentyfive milligrams were finely ground and evenly suspended by sonifying for 15 sec in 5 ml peanut oil. The dosage was 0.25 mg/bird/day (= 10 mg/ kg). The suspension was inoculated intramuscularly in the breast or leg with a disposable 1-ml syringe through a 20-gauge needle. Control birds received an equivalent amount (0.05 ml) of peanut oil only. Sparrows used in this study were part of a field population experimentally infected with P. relictum at State College, Pa. The birds were captured, banded, inoculated with parasitized whole blood, and released. Individuals used in this study were recaptures which had been infected for at least 125 days. Twelve birds were recaptured, returned to the laboratory, and divided randomly into 2 groups of 6 birds each. One group received the daily dose of corticosterone and the other the oil vehicle. Treatment was continued for 10 days and blood films were collected at 48-hr intervals for 16 days. The basic experiment was repeated at 6-week intervals for three replicates commencing on 22 January, 4 March, and 14 April, respectively. The chi-square method was used for statistical analysis of data.