Summary. Mice were exposed to 5 h of restraint stress on Days 1\p=n-\3,4\p=n-\6, or 1\p=n-\6of pregnancy in the morning (08:30\p=n-\13:30h, a.m.) or afternoon (13:30\p=n-\18:30h, p.m.). Stress reduced the pregnancy rate from 90 to 52% (P < 0\m=.\005) and average litter size on Day 18 from 8\m=.\2to 5\m=.\2 young (P < 0\m=.\005). Stress for 6 days was more effective than for 3 days (P < 0\m=.\005) and an a.m. stress was more effective than a p.m. stress (P <0\m=.\05)