Pregnant rats were treated either throughout gestation (GD 1–20) with 30 mg/kg per day (chronic cocaine) or with one 15-mg/kg dose immediately following parturition (acute cocaine). Chronic and acute cocaine treatment delayed or diminished the postpartum onset of some components of maternal behavior, and chronically treated dams were significantly more aggressive toward a male intruder than acute cocaine-treated or saline-treated dams. Cocaine increased the latency to crouch over pups and decreased crouch duration during a 30-min observation period that immediately followed parturition. Latencies to nest build were also longer in more chronic cocaine-treated dams than in saline controls. On Day 6 postpartum, 83% of chronic cocaine-treated dams pinned and attacked an intruder male 8 or more times during a 10-min observation period, whereas only 4% of acute cocaine-treated and none of the saline-treated dams exhibited this much aggression.