The nature of the structural regression of the corpus luteum of pregnancy (PCL) was investigated in rats from day 19 of pregnancy (PRG19) through day 6 postpartum (PP6). A pre-partum decrease in tissue weight of the PCL during the last 4 days of pregnancy was apparent (nearly a 40% decrease) and seemed to parallel the fall in plasma progesterone. The PCL in normally lactating rats further experienced a gradual decrease in the postpartum period, but the reduction was significantly slowed by PP6 in non-lactating rats that were forced to wean from the parturient day (PP0). TUNEL analysis revealed that apoptotic cell death in the regressing PCL showed a 3–4 fold increase in signals after parturition (on PP3) but was fairly limited. Instead, luteal steroidogenic cells came to have cytoplasmic vacuoles by PRG21, and vacuoles increased through PP3. A decelerated decrease in PCL weight following forced weaning was potentially reversed by repeated administration of exogenous prolactin (10 IU) twice daily. This study adds evidence that structural regression of the rat PCL 1) starts to occur before the completion of functional regression, 2) involves a modest incidence of TUNEL-reactive apoptosis, and 3) is promoted by the suckling stimulus, presumably through the effects of prolactin.