Sex, age and shock-intensity were studied in a step-through passive avoidance test. In adult rats (140 days) males showed more inhibition of a previously punished approach response. No sex differences were observed in young adult animals (60 days). In prepuberal animals (30 days), on the other hand, it was the females which showed more response inhibition. In prebuberty females showed far more response inhibition than in adulthood. Even when using 1.6 mA as a reinforcer, only 17% of the adult females showed inhibition after punishment of the entering response.