Several parameters on pregnancy and parturition of primiparous Sprague-Dawley rats were examined. Interval between placing with males and pregnancy averaged 7.4 +/- 1.1 days and 75% of rats became pregnant within 10 days after mating (Fig. 1). Presence of sperms in the smears showed much higher pregnancy ratio than the other two indices, vaginal plug and the last estrus (Table 1). Pregnancy period was mostly 23 days when the day of appearance of each copulation signs was designated as 1 day of pregnancy (Table 2) and all rats started parturition during light time of animal room (7:00 AM approximately 9:00 PM) on both 22 and 23 days of pregnancy (Fig. 2). Average time required for parturition was 105 +/- 10 minutes and mostly less than 2 hours (Fig. 3). There was no correlation between litter size (12.6 +/- 0.5 : Fig. 4) and the time required for parturition. The ratio of still-born pups was 1%.