Albino ICR mice, 384 females and 48 males, were allocated to control or stress treatments. For the latter, animals were exposed to 34·5° C and 65% relative humidity for a period of 24 hr. The treatment of males commenced 48 hr before a 6-day mating schedule and of females, at 16.00 hours on the day a copulatory plug was detected. The females were killed 54 hr or 10 days after a plug had been detected. The developmental stages of embryos recovered at 54 hr were recorded and four-cell and eight-cell ova were incubated in [3H]uridine and subjected to autoradiography. Females killed at Day 10 of gestation gave estimates of pre- and postimplantation embryonic mortality.