The present study examined ontogeny of behavioral effect of acute cocaine administration and behavioral sensitization to a challenge dose of cocaine in rats previously treated with chronic cocaine. Acute stimulating effects of cocaine in behaviors were observed in as early as 7-postnatal-day (PND) rat. However, they needed higher dose of cocaine than adult rats. Adult pattern of response to acute doses of cocaine was seen in stereotypy and locomotion at 21 PND, and in rearing at 28 PND. Rats at ages of 7, 14, 21 or 28 PNDs received repeated injections of either saline or cocaine 15 mg/kg twice a day for five consecutive days. After an abstinent period of three weeks, sensitization to a challenge dose of cocaine (15 mg/kg) was assessed. Cocaine-induced stereotyped behaviors were significantly enhanced only when cocaine-pretreatment was initiated at PND 21 and 28, but not at the earlier ages of PND 7 or 14. These results suggest that cocaine-induced behavioral sensitization occurs only when subchronic cocaine administration was commenced after PND 21.