Cannabinoids have potent vasodilatory actions in a variety of vascular preparations. Their mechanism of action, however, is complex. Apart from acting on vascular smooth muscle or endothelial cannabinoid receptors, several studies point to the activation of type 1 vanilloid (TRPV1) receptors on primary afferent perivascular nerves, stimulating the release of calcitonin gene-related peptide (CGRP). In the present study, the direct influence of the cannabinoid methanandamide and the neuropeptide CGRP on the membrane potassium ion (K(+)) currents of rat mesenteric myocytes was explored. Methanandamide (10 μM) decreased outward K(+) currents, an effect similar to that observed in smooth muscle cells from the rat aorta. Conversely, CGRP (10 nM) significantly increased whole-cell K(+) currents and this effect was abolished by preexposure to tetraethylammonium chloride (1 mM) or iberiotoxin (100 nM), inhibitors of large-conductance calcium-dependent K (BK(Ca)) channels but not by glibenclamide (10 μM), an inhibitor of ATP-dependent K channels. In the presence of the CGRP receptor antagonist CGRP(8-37) (100 nM), the adenylyl cyclase inhibitor SQ22536 (100 μM), or the protein kinase A inhibitor Rp-cAMPS (10 μM), CGRP had no effect. These findings show that methanandamide does not increase membrane K(+) currents in smooth muscle cells of small mesenteric arteries, supporting an indirect mechanism for the reported hyperpolarizing influence in this vessel. Moreover, CGRP acts directly on these smooth muscle cells by increasing BK(Ca) channel activity in a CGRP receptor and cyclic adenosine monophosphate-dependent way. Collectively, these data indicate that methanandamide relaxes and hyperpolarizes intact mesenteric vessels by releasing CGRP from perivascular nerves.