Pregnant Swiss albino mice were exposed to 0.3, 0.5, 1.0, or 1.5 Gy of gamma radiation on day 17 of gestation. Sham-exposed controls were examined for comparison. Exposed mice as well as controls were left to complete gestation and parturition. Pups were observed up to age 6 weeks; appearance of physiological markers (pinna detachment, eye opening, fur development, vaginal opening, and testes descent), postnatal mortality, body weight, body length, head length, head width, and tail length were recorded. A significant delay in fur development was observed at 0.3 Gy and in other physiological markers at doses above 0.3 Gy, while a significant increase in mortality and growth retardation occurred only at 1.0 and 1.5 Gy. Although congenital anomalies such as syndactyly and bent tail were observed at doses of 0.5-1.5 Gy, only syndactyly showed a statistically significant increase in frequency. A statistically significant lower body weight was observed during the first week of postnatal life, but body weights increased to normal levels by the second week in animals exposed to doses less than 1.0 Gy. At higher doses, low body weight persisted throughout the postnatal period. Head length and tail length showed a significant decrease from controls at 0.5-1.5 Gy, and the effect was evident from birth to age 6 weeks. But a similar effect on body length and head width was noticed only at 1.0 and 1.5 Gy. These studies indicate that even in the absence of any major morphological changes, normal development of physiological landmarks and postnatal growth can be impaired by fetal irradiation at 17 days p.c. (post coitus). Morphological changes appear to have a threshold between 0.3-0.5 Gy, while physiological marker effects may occur with a lower threshold. Teratology 59: 133-138, 1999.