We examined whether blockade of prostaglandin synthesis by indomethacin could attenuate the effect of nitric oxide synthase (NOS) inhibition on cerebral arteriolar dilation during cortical spreading depression (CSD). CSD was induced by microinjection of 5% (670 mM) KCl onto the cerebral cortex of anesthetized adult rabbits. A closed cranial window and intravital microscopy were used to measure pial arteriolar diameter, and NOS activity was determined by the conversion assay of [14C]arginine to [14C]citrulline. CSD dilated pial arterioles by 47 +/- 3% (baseline = 80-88 microns) (n = 21, P < 0.05), and inhibition of NOS by NG-nitro-L-arginine (L-NNA) (15 mg/kg iv) reduced dilation during CSD by over one-half (n = 8, P < 0.05) without altering the onset latency to CSD. After indomethacin administration (15 mg/kg iv), CSD dilated arterioles from 73 +/- 2 to 152 +/- 6 microns (n = 4, P < 0.05). However, after administration of both indomethacin and L-NNA (n = 5), CSD-induced arteriolar dilation was not different from the situation where indomethacin alone was given. Thus indomethacin completely abolished the inhibitory effect of L-NNA on CSD-induced dilation. Administration of L-NNA inhibited NOS activity in brain cortex almost completely (n = 8, P < 0.05), whereas indomethacin itself had no effect (n = 8). In addition, L-NNA inhibited topical acetylcholine (10(-5) M)-induced arteriolar dilation (n = 3, P < 0.05), and this effect was not altered by indomethacin (n = 4). In summary, L-NNA reduced arteriolar dilation during CSD. However, after administration of indomethacin, L-NNA does not reduce CSD-induced arteriolar dilation.