NITRIC oxide (NO) is becoming recognized as an important intercellular messenger in the brain. The present experiment used microdialysis to examine the potential role of NO in the regulation of dopamine (DA) and serotonin (5-HT) release in the medial preoptic area (MPOA) of freely moving male rats. The NO precursor L-arginine (L-Arg, 100 μM), administered into the MPOA via the dialysis probe, increased extracellular levels of DA, 5-HT, and the major metabolites of DA. These increases were blocked by the coadministration of the NO synthase inhibitor N-monomethyl L-arginine (NMMA, 400 μM). The inactive isomer D-arginine (100 μM) was ineffective, and NMMA by itself decreased DA below baseline levels. Thus, NO may modulate the release of DA and 5-HT in the MPOA.