The growth of 22 litters of the Mongolian gerbil (Meriones unguiculatus) was studied for 15 weeks. Mean weight at birth was 2.8 ? S.D. 0.28 g. At sexual maturity (about week 10) males averaged 54.4 + S.D. 5.53 g and females, 46.7 + S.D. 6.20 g. Increase in weight continued for at least 1 year, but beyond week 15 much of this was the result of fat deposition. After week 5, males were significantly heavier than females. Increase in total, body, and tail length was nearly complete by week 15, and growth of the hind foot and ear was negligible after week 5-6. Males were consistently larger than females in linear measurements after week 4. A descriptive account of postnatal pelage development is given. Hair development was rapid and by the time weaning occurred on about day 21 the juvenile pelage was completed. The postjuvenile and postsubadult molts were studied using Nyanzol A dye and averaged 17.0 and 38.5 'days in duration, respectively. First indications of the postjuvenile molt were seen between 32 and 38 days of age (mean 35.4 days, N = 9), and the postsubadult molt began between days 58 and 63 (mean 60.9, N = 8). INTRODUCTION The Mongolian gerbil, Meriones unguiculatus, is a hamster-sized rodent of the family Cricetidae, subfamily Gerbillinae. Originally native to the desert and semiarid regions of Mongolia and northern China (Allen, 1940; Kowalski, 1968; Walker, 1968), M. unguiculatus was imported to the United States in 1954 (Schwentker, 1963, 1968). Since then, it has become increasingly important as a laboratory animal and has been used extensively as a test subject in a variety of biomedical and nutritional studies (see Rich, 1968, for a bibliography of this literature). Despite its wide use in the laboratory, however, few details of its growth and development are available. Schwentker (1963) and Marston and Chang (1965) found that the average litter size is 4.5, birth weight is 2.5 to 3.5 g, weaning takes place at 3 weeks (11 to 18 g), and sexual maturity occurs at 9 to 12 weeks (about 40 g). Feldman and Mitchell (1968) studied hair development in male gerbils histologically, and McManus (in press) studied early postnatal growth and development. The purpose of the present study is to determine growth rate, pelage development and molt progression in M. unguiculatus. METHODS AND MATERIALS The animals used in this study were the offspring of 10 breeding pairs obtained from Tumblebrook Farm, Inc., Brant Lake, N.Y. Because the Tumblebrook Farm colony stems from five males and four females imported in 1954, and because it has been bred for uniformity