Rat pups were isolated from the mother and nest for 1 hr per day from Postnatal Day (PN) 2 to 9 At PN 27, rats were tested for behavioral responsiveness to 2.0 or 7.5 mg/kg amphetamine. Only isolated rats receiving the 7.5 mg/kg dose displayed increased activity scores, compared with nonisolated and nonhandled controls. Their increased activity is attributed to a slower latency to enter into stereotypy. In a second experiment, similarly treated groups were challenged by the 7.5 mg/kg dose during a session in which a microdialysis probe implanted in the ventral striatum was being perfused. The challenge drug elicited a much greater increase in dialysate dopamine in isolated vs. nonisolated groups. Results are discussed with regard to dissociation between sensitized and subsensitized responses.