We used mice with deletions in either the endothelial nitric oxide synthase (eNOS) or neuronal NOS (nNOS) gene to investigate the role of eNOS and nNOS in acetylcholine (ACh)-induced relaxation of pial arterioles (20-30 microns). Pial arteriolar diameter was measured by intravital microscopy through a closed cranial window, and NOS activity was determined by the conversion of [3H]arginine to [3H]citrulline in subjacent cortex. ACh superfusion (1, 10 microM) caused atropine-sensitive dose-dependent arteriolar dilation in all three mouse strains. At 10 microM, increases of 20 +/- 2, 31 +/- 3, and 23 +/- 3% were recorded in wild-type (n = 25), nNOS mutant (n = 15), and eNOS mutant (n = 20) mice, respectively. NG-nitro-L-arginine (L-NNA, 1 mM) superfusion inhibited cortical NOS activity by > 70% and abrogated the response in wild-type mice while blocking the dilation by approximately 50% in eNOS mutant and nNOS mutant mice. Only in the eNOS mutant did tetrodotoxin (TTX) superfusion (1 microM) attenuate ACh-induced dilation (n = 6). The residual dilation after L-NNA in eNOS mutant mice could be blocked completely by TTX-plus L-NNA. Our findings indicate that 1) ACh dilates pial arterioles of wild-type mice by NOS-dependent mechanisms as reported in other species, 2) the response in nNOS mutant mice resembles the wild-type response except for enhanced dilation to ACh and reduced L-NNA sensitivity, and 3) surprisingly, the response in eNOS mutant mice is partially NOS dependent and attenuated by both TTX and L-NNA. Because nNOS is constitutively expressed in eNOS mutants, these findings coupled with the TTX results suggest that an nNOS-dependent mechanism may compensate for the chronic loss of eNOS activity after targeted gene disruption.