Dopamine receptors have been categorized into subfamilies D1 and D2, each with separate roles in dopamine-mediated behaviors. Of the D2 subfamily, the dopamine D3 receptor has been cloned, but the behavioral effects of selectively stimulating the D3 receptor are largely unknown. The purpose of this study was to quantify the locomotor responses of developing rats to the putative dopamine D3 receptor agonist, 7-hydroxy-N,N-di-n-propyl-2-aminotetralin (7-OH-DPAT). One of three doses of 7-OH-DPAT (0.01, 0.10, 1.00 mg/kg) or saline was injected subcutaneously into rats at the age of 10,20,30, or 60 days. Five minutes after the injection, rats were placed in automated activity monitors which recorded locomotor behavior at 5 min intervals for 2 h. The high dose of 7-OH-DPAT increased locomotor activity in rats of all ages. The medium and low doses increased activity in 10- and 20-day-old rats but not in 30- or 60-day-old rats. The level of drug-induced activation peaked at 20 days of age. In 30- and 60-day-old rats, but not 10- and 20-day-old rats, a period of locomotor suppression preceded the activation in response to the high dose of 7-OH-DPAT. In rats aged 20 days and older, the middle and low doses decreased locomotion early in the test session, but activation did not ensue. This dose-response pattern across ontogeny closely resembles that induced by quinpirole, an agonist at the dopamine D2 receptor subfamily.