Apoptosis is associated with regression of corpora lutea (CL) in a number of species. The present work compared apoptotic rates, assessed by low molecular weight DNA labelling, in rat CL of the oestrous cycle, pregnancy, the immediate post-partum period, and in vitro culture. Apoptosis was significantly related to cycle phase with peak activity at oestrus. This pattern was similar in CL formed from the most recent ovulation and from the two previous generations. Apoptosis was evident during pregnancy, albeit at low rates. It declined on the day of parturition and increased on the following day. Apoptosis was greatly increased in cultured CL to reach 20-fold higher levels than those achieved during the oestrous cycle or pregnancy. Collectively, these results suggest that apoptosis has lesser significance in rat CL functional regression than in other species with more precipitate structural changes. However, rat CL clearly retain the potential for high rates of apoptosis and so present a useful model for examining molecular mechanisms of apoptotic cell death.