Cerebral vasodilator responses are often impaired following subarachnoid hemorrhage (SAH). Because depolarization of vascular muscle may occur after SAH, we tested in vivo the hypothesis that SAH may augment dilatation in response to hyperpolarization due to activation of K+ channels. Anesthetized rats were studied two days after injection of saline or autologous blood into the cisterna magna. Diameter of the basilar artery in vivo was 224 +/- 5 microns (mean +/- SE) in saline-treated rats and 201 +/- 6 microns in SAH rats (P < 0.05). In control rats, acetylcholine (ACh), sodium nitroprusside (SNP), aprikalim and calcitonin gene-related peptide (CGRP; both activators of ATP-sensitive K+ channels), papaverine, 8-bromoguanosine 3',5'-cyclic monophosphate (8-BrcGMP), and brain natriuretic peptide (BNP; an activator of particulate guanylate cyclase) produced concentration-dependent dilatation. In SAH rats, vasodilatation was impaired in response to ACh and SNP. In contrast, vasodilator responses to aprikalim and CGRP were augmented in SAH, rats (by two- to fourfold). Vasodilator responses to 8-BrcGMP, papaverine, and BNP were similar in both groups. Thus responses mediated by activation of soluble guanylate cyclase are selectively impaired by SAH, but responses to guanosine 3',5'-cyclic monophosphate are normal. Vasodilator responses to activation of ATP-sensitive K+ channels are augmented by SAH.