1. The vasorelaxant effect of the sodium salt of the short chain fatty acid, butyrate, on preconstricted rat small mesenteric arteries (mean inner diameter approximately 300 microns) was characterized. Isometric force development was measured with a myograph, and intracellular pH (pHi) was simultaneously monitored, in arteries loaded with the fluorescent dye BCECF in its acetomethoxy form. Sodium butyrate (substituted isosmotically for NaCl) was applied to arteries after noradrenaline (NA) or high K+ contractures were established. 2. Arteries preconstricted with a concentration of NA inducing an approximately half maximal contraction were relaxed by 91.5 +/- 6.3% by 50 mmol l-1 butyrate. This concentration of butyrate did not, however, cause a significant relaxation of contractures to a maximal (5 mumol l-1) NA concentration, and also failed to relax significantly contractures stimulated by high (45 and 90 mmol l-1) K+ solutions. Contractures elicited with a combination of NA (at a submaximal concentration) and 45 mmol l-1 K+ were, however, markedly relaxed by butyrate. 3. Investigation of the concentration-dependency of the butyrate-induced relaxation of the half maximal NA response revealed an EC50 for butyrate of approximately 22 mmol l-1. 4. Sodium butyrate (50 mmol l-1) caused pHi to decrease from 7.25 +/- 0.02 to 6.89 +/- 0.08 (n = 4, P < 0.001). However, the vasorelaxant effect of butyrate on the submaximal NA contracture was not significantly modified when this fall in intracellular pH was prevented by the simultaneous application of NH4Cl. 5. Butyrate-induced relaxation was also unaffected by endothelial denudation and inhibition of NO synthase with N omega-nitro-L-arginine methyl ester (100 mumol l-1). 6. The relaxation of the NA contracture by 50 mmol l-1 sodium butyrate was abolished in arteries pretreated with the cyclic AMP antagonist Rp-cAMPS (25 mumol l-1). 7. We conclude that the butyrate-induced relaxation of the NA contracture is independent of intracellular acidification. The ability of Rp-cAMPS to abolish the butyrate relaxation indicates that stimulation of the cyclic AMP second messenger system may play an important role in mediating this effect.