Apomorphine-induced locomotor stimulation was investigated in the developing rat following injection with 6-hydroxydopa at birth. Treatment with 6-hydroxydopa potentiated locomotor responsiveness to apomorphine in the 20-day-old rat. The 6-hydroxydopa-treated animal at 30 days, however, was less sensitive to the drug than was the control. Apomorphine again elicited more locomotor stimulation in 6-hydroxydopa-treated animals than in the controls on day 50. These results suggest that the altered sensitivity of dopamine receptors induced with 6-hydroxydopa, is influenced by the onset of activity of other "inhibitory" neurons on day 30.