Effects of the noncompetitive N-methyl-D-aspartate (NMDA) channel blocker, MK-801, on play behavior and activity were assessed in juvenile rats. A low dose (0.025 mg/kg) of MK-801 significantly increased both pinning and dorsal contacts, two indices of play behavior in the rat. Higher doses (> or = 0.1 mg/kg) significantly reduced both measures of play, with play being virtually abolished at 0.2 mg/kg. Simultaneous measurements of activity indicated that the reductions of play at the higher doses were accompanied by significant increases in horizontal activity. There was also a dose-related reduction in rearing, which was significant at all doses. When juvenile rats were tested individually for activity in an open field, MK-801 increased horizontal activity at 0.2 mg/kg and decreased rearing at all doses. So while the effects of MK-801 on play are biphasic, the effects of this compound on locomotor activity, per se, are linear. These data suggest that the effects of NMDA channel blockade on overall activity and play are mediated through independent neural systems.