Ca²⁺ entry via L-type voltage-gated Ca²⁺ channels (LVGCs) is a key factor in generating myogenic tone (MT), as dihydropyridines (DHPs) and other LVGC blockers, including Mg²⁺, markedly reduce MT. Recent reports suggest, however, that elevated external Mg²⁺ concentration ([Mg²⁺]_o) and DHPs may also inhibit other Ca²⁺ entry pathways. Here, we explore the contribution of LVGCs to MT in intact, pressurized mesenteric small arteries using mutant mice (DHP^R/R) expressing functional but DHP-insensitive Ca_v1.2 channels. In wild type (WT), but not DHP^R/R, mouse arteries, nifedipine (Nif, 0.3-1.0 µM) markedly reduced MT as well as vasoconstriction induced by high external K⁺ concentrations ([K⁺]_o), a measure of LVGC-mediated Ca²⁺ entry. Block of MT and high [K⁺]_o-induced vasoconstriction by 1 µM Nif in WT, but not DHP^R/R, arteries implies that Ca²⁺ entry via Ca_v1.2 LVGCs is obligatory for MT, and that Nif inhibits MT exclusively by blocking LVGCs. We also examined the effect of Mg²⁺ on MT and LVGCs. High [Mg²⁺]_o (10 mM) blocked MT, slowed the high [K⁺]_o-induced vasoconstrictions and decreased their amplitude in WT and DHP^R/R arteries. To verify that these effects of Mg²⁺ are due to block of LVGCs, we characterized the effect of extracellular and intracellular Mg²⁺ on LVGC currents in isolated mesenteric artery myocytes. DHP-sensitive LVGC currents are inhibited by both external and internal Mg²⁺. The results indicate that Mg²⁺ relaxes MT by inhibiting Ca²⁺ influx through LVGCs. These data provide new information about the central role of Ca_v1.2 LVGCs in generating and maintaining MT in mouse mesenteric small arteries.