Breeding and pregnancy in rhesus monkeys used for teratological testing

Fifty‐two young rhesus females were bred intensively under laboratory conditions for 6‐24 months. More than half became pregnant by 90 days after first observed menstruation, and all of an original group of 37 became pregnant by 2 years. A rate of 1.2 pregnancies per female per year was achieved by interrupting pregnancy by hysterotomy at day 100 and rebreeding the female within 1 to 2 months. Further improvement on this rate is a reasonable expectation. After hysterotomy menstruation usually resumed by 30–50 days. Second pregnancies occurred in 80% of 28 females by 9 months and in all by 16 months. Accurately anticipating the time of ovulation is unresolved, but estimation of chloride content of cervical mucus has permitted more effective breeding in some cases than would have been possible by routine breeding on days 11‐13 of the menstrual cycle. Clomiphene citrate did not improve conception rate in relatively infertile animals above that observed in untreated animals with similar reproductive histories. A biological pregnancy test was developed that can be completed in 24 hours, can diagnose pregnancy as early as day 16, and is highly reliable by day 20 of gestation. Early abortions after teratological tests were diagnosed by uterine palpation. The 100‐day fetus proved entirely satisfactory for evaluation of anatomical development.