The extracellular serine protease tissue plasminogen activator (tPA) that converts plasminogen into plasmin is abundantly expressed throughout the central nervous system. We have recently demonstrated that the tPA-plasmin system participates in the rewarding and locomotor-stimulating effects of morphine by acutely regulating morphine-induced dopamine release in the nucleus accumbens (NAc). In the present study, we examined the effects of microinjections of plasminogen activator inhibitor-1 (PAI-1), tPA or plasmin into the NAc on morphine-induced dopamine release, hyperlocomotion and anti-nociceptive effects in ICR mice. A single morphine treatment resulted in an increase in protein levels of PAI-1 in the NAc. Microinjection of PAI-1 into the NAc dose-dependently reduced morphine-induced dopamine release and hyperlocomotion. In contrast, microinjection of tPA into the NAc significantly potentiated morphine-induced dopamine release and hyperlocomotion without affecting basal levels. Furthermore, microinjection of plasmin enhanced morphine-induced dopamine release, but did not modify the hyperlocomotion induced by morphine. The intracerebroventricular injection of PAI-1, tPA and plasmin at high doses had no effect on the anti-nociceptive effects of morphine. These results suggest that the tPA-plasmin system is involved in the regulation of morphine-induced dopamine release and dopamine-dependent behaviors but not the anti-nociceptive effects of morphine.