A method for regulating the duration of pregnancy and the time of parturition in Sprague-Dawley rats (Charles River CD strain).

A method of shifting the dark phase of the 12/12 hr photoperiod cycle on Day 7 of pregnancy is described for regulating the duration of pregnancy to 22 days and assuring that most parturitions take place between 1100 and 1500 hr. This method, in addition, has the added convenience that matings can occur during normal laboratory hours between 0700 and 1900 hr during the dark phase of the photoperiodic cycle. Litter sizes are normal (mostly 14-16 pups) and postpartum estrus behavior occurs at the normal interval of 4 to 11 1/2 hr after parturition.