Department of Gynecology and Obstetrics, and Human Development Center, University of Kansas Medical Center, Kansas City, Kansas 66103, U.S.A. (Received 18 December 1974) Knowing the exact age of the corpus luteum (CL) may be of importance in studies designed to assess the factors that influence CL survival and luteolysis. Control of the CL in vitro, from the moment of ovulation until luteolysis, is an objective of a sequence of experiments utilizing organ cultures (Fainstat & Emanuele, 1972; Baranczuk, Cheatum & Fainstat, 1975) and organ incubations in this laboratory. The accurate determination of CL age, based upon the time of ovulation in vitro, is desirable for correct interpretations of data from our in-vitro experiments. The purpose of this present study was to determine the influence of time on the likelihood of ovulation in vitro. Identifying the time of the peak incidence of ovulations would aid the selection of the optimal time