Nitric oxide (NO) contributes to hypoxia-induced pial artery dilation, at least in part, via the formation of guanosine 3',5'-cyclic monophosphate (cGMP) and subsequent release of Met-enkephalin and Leu-enkephalin in the newborn pig. In separate studies, these opioids were also observed to elicit NO-dependent pial dilation. The present study was designed to investigate the role of the neuronal isoform of NO synthase (NOS) in hypoxic pial dilation, associated opioid release, and opioid dilation in piglets equipped with a closed cranial window. Tetrodotoxin (10⁶ M) attenuated the dilation resulting from hypoxia (PO₂ ~35 mmHg; 25 ± 1 vs. 14 ± 1%). Similarly, 7-nitroindazole, sodium salt (7-NINA, 10⁶ M), a purported neuronal NOS inhibitor, attenuated hypoxic pial dilation (26 ± 1 vs. 14 ± 2%). Hypoxic dilation was accompanied by elevated cerebrospinal (CSF) cGMP, which was blocked by 7-NINA (433 ± 19 and 983 ± 36 vs. 432 ± 19 and 441 ± 19 fmol/ml for control and hypoxia in absence and presence of 7-NINA, respectively). Additionally, hypoxic dilation was also accompanied by elevated CSF Met-enkephalin, which was attenuated by 7-NINA (1,027 ± 47 and 2,871 ± 134 vs. 779 ± 78 and 1,551 ± 42 pg/ml for control and hypoxia in absence and presence of 7-NINA, respectively). In contrast, Met-enkephalin (10¹⁰, 10⁸, and 10⁶ M) induced dilation that was unchanged by 7-NINA (7 ± 1, 12 ± 1, and 18 ± 1 vs. 6 ± 1, 10 ± 1, and 17 ± 1%, respectively). N-methyl-D-aspartate (NMDA, 10⁸ and 10⁶ M), an activator of neuronal NOS, induced pial dilation that was blocked by 7-NINA (10 ± 1 and 20 ± 2 vs. 1 ± 1 and 2 ± 1%, respectively). However, sodium nitroprusside-induced dilation was unchanged by 7-NINA. These data indicate that neuronal NOS contributes to hypoxic pial artery dilation but not to opioid-induced dilation. Furthermore, these data suggest that neuronally derived NO contributes to hypoxic dilation, at least in part, via formation of cGMP and the subsequent release of opioids.