Abstract
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 copulaiton sign was designated as 1 day of pregnancy (Table 2) and all rats started parturition during light time of animal room (7 : 00 AM 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%.
