The relative role of central mu-opioid receptor agonists Tyr-D-Ala-Gly-N-Methyl-Phe-Gly-ol (DAMGO), Tyr-D-Arg-Phe-Lys (DALDA), and Tyr-Pro-MePhe-D-Pro (PL017) (0.00, 0.01, 0.1, or 1.0 mu g, ICV) on behavior was investigated in rats for 60 min in activity monitors. DAMGO (0.1 and 1.0 mu g) and PL017 (1.0 mu g) resulted in biphasic effects, inhibition followed by hyperactivity for linear locomotor, whereas the 0.01-mu g dosage was associated with hyperactivity. On the other hand, DALDA (0.1 and 1.0 mu g) suppressed locomotor activity over the 60-min session.