Rabbits ovariectomized 24 hr post coitum (p.c.) were injected i.m. with 2 mg or 4 mg 17α-hydroxyprogesterone caproate daily. Pre-natal development was compared with that of control litters at 9 and 29 days, 5 days and 7 days p.c. The 2 mg treatment supported 26·4% pre-natal survival to 29 days p.c., most mortality occurring between 7 and 9 days. The 4 mg dose rate permitted 71 ·0% survival to 29 days p.c. which was not significantly different from controls (81·5%). Blastocyst size at 5 days p.c. was not significantly affected by treatment but both ovariectomized groups showed slight retardation at 7 days when stage of attachment was assessed by observation and by analysis of blastocyst fluid for lactic acid. Attachment was least advanced in the group receiving the lower dose rate.
In 29-day foetuses the degree of ossification of the femur was positively related to foetus weight. In control litters deposition of calcium and total ash components and relative decrease in water content were also further advanced in the larger foetuses, suggesting a general association of size and maturity.