Groups of pseudopregnant rats were injected intravenously with (-)-trans- delta 9-tetrahydrocannabinol (THC) to determine its effects on serum prolactin (PRL) and the maintenance of pseudopregnancy. A single injection of 4 mg THC/kg BW at 2400 h on the first day of leukocytic vaginal smears of pseudopregnancy (D-1) delayed the ensuing nocturnal PRL surge for approximately one hour. When THC (1.0 mg/kg BW) was administered hourly from 2400 h on D-1 through 1700 h on D-2, the nocturnal surge was blocked and serum PRL levels were suppressed until 0600 h on D-2, but not thereafter. Neither treatment altered the duration of pseudopregnancy. These results indicate that the nocturnal surge secretion of PRL during early pseudopregnancy in the rat is sensitive to THC suppression, but that this suppression is not adequate to influence the duration of pseudopregnancy. The mechanism through which THC exerts this action remains unknown.