We have previously shown that interleukin-1β relaxes vascular smooth muscle by the NO-dependent and independent mechanisms (Takizawa et al.: Eur. J. Pharmacol. 330: 143-150, 1997). In this study, we investigated the mechanism of NO-independent relaxation. Treatment of the rat aorta with interleukin-1β for 24 hr inhibited the high-K⁺ induced contraction by decreasing cytosolic Ca²⁺ level ([Ca²⁺]_i). The relationship between [Ca²⁺]_i and tension in intact muscle and the pCa-tension curves in permeabilized muscle suggested that Ca²⁺ sensitivity of contractile element was not changed after the interleukin-1β-treatment. After a treatment with interleukin-1β for 24 hr, contractile effects of phenylephrine (1 μM-10 μM) were markedly inhibited in the presence of L-NMMA (100 μM) applied to inhibit NO synthesis. A blocker of ATP-sensitive K⁺ channel, glibenclamide (1 μM), partially recovered the interleukin-1β-induced inhibition. In contrast, a blocker of Ca²⁺-activated K⁺ channel, charybdotoxin (0.1 μM), was ineffective. These results suggest that membrane hyperpolarization due to activation of ATP-sensitive K⁺ channels may partly be responsible for the NO-independent mechanism of interleukin-1β-induced inhibition of vascular smooth muscle contraction.