Freshly isolated rat circulating neutrophils (PMN) constitutively expressed neural nitric oxide synthase (nNOS) mRNA and nNOS protein and exhibited spontaneous basal release of low concentrations of nitrate and nitrite anion (RNI). In contrast, rat peripheral monocytes and macrophages were devoid of nNOS mRNA and protein and did not exhibit basal or spontaneous release of RNI. Constitutive neural NOS mRNA was also found in human PMN. However, nNOS protein was not expressed and spontaneous generation of RNI was absent in the human PMN. Spontaneous release of RNI from rat PMN was inhibited by 7-nitroindazole but not by L-N-iminoethyllysine, which further supported the nNOS origin of the spontaneously produced RNI. Intravenous administration of Escherichia coli endotoxin (0.6 mg/kg) did not acutely affect the content of nNOS mRNA or protein but inhibited nNOS-derived production of RNI in PMN and up-regulated iNOS mRNA and iNOS protein in PMN, macrophages, and monocytes. This communication demonstrates the existence of nNOS mRNA in rat and human PMN and nNOS protein in rat PMN. Moreover, the data also show that the nNOS system in rat PMN is functional and is inhibitable by the nNOS inhibitor 7-nitroindazole. These findings offer an explanation for the spontaneous formation of the PMN-derived relaxing factor resembling nitric oxide (NO). Moreover, since basal production of NO can affect expression of adhesion molecules and cell-cell binding, the nNOS system within the rat may play an important role in PMN function in normal and disease states. Finally and speculatively, if constitutively expressed nNOS mRNA is subject to activation and translation into nNOS protein, nNOS may also play a role in the function of human PMN.