The purpose of this investigation was co discover whether che 22-day-old rat pup would display any consistant relative preference, i.e., discrimination, for other members of ics species. Two dimensions of possible social incentives were investigated: familiar versus unfamiliar animals; age-mates versus adults. Ten pups served as Ss, six from a litter of 12 and 4 from a litter of 10. The mothers had been removed from the litter cages when the pups were 21 days old. All animals were on ad libitum feeding prior to testing. Mothers and remaining litter-mates as a group served as familiar incentives. Two additional mothers and comparable numbers of their pups served as unfamiliar incentives. Each of the four incentives was paired against each of the others in the end boxes of a short Y-maze. They were visible from the start box when the door was raised. Thus, for each S chere were six incentive pairings. Ten trials were given for each pairing, the positions of the incentives being a1ternated.l The order of pairings was random for the first 6 Ss and this order was reversed for the last 4 Ss. Relative preference scores were calculated by adding all runs to each incentive over all pairings. Since each incentive appeared in three pairings, the total possible was 30 wich 15 expected by chance. The resulting means were: familiar female (mother), 12.0; familiar agemates (siblings), 14.6; unfamiliar female, 15.7; and unfamiliar age-mates, 17.7. Lack of independence of these means precluded direct comparison. Deviations from the expected 15, however, could be tesced and were significant for the familiar female (t = 3.50, df = 9, p < .01) and for the unfamiliar agemates (t = 5.22, df = 9, p < .01). These tests give a fair indication that the results are not due to chance. Clearly, 22-day-old rat pups can discriminate among individuals of their own species. With respect to the incentive dimensions, age-maces appear to be preferred over adults and unfamiliar over familiar animals. A tentative two-factor interpretation is offered. Observation of pre-weaned litters suggests that litter-mates are a greater source of contact comfort than is the morher. Consequently, as Harlow and Zimmerrnann's monkeys (1759) responded to cloth surrogates in preference to lactating wire surrogates, so do rat pups respond to age-mates (cf. Scorn, 1758, pp. 184-5). The preference for unfamiliar animals is taken as representing the curiosity phenomenon. REFERENCES HARLOW, H. F., & ZIMMERMANN, R. R. Affectjonal responses in the infant monkey. Science, 1957, 130, 421432. SCOTT, J. P. Animal behavior. Chicago: Univer. of Chicago Press, 1758.