The effect of Tris on Ca2+ accumulation by microsomal membrane fraction isolated from rat vas deferens was studied. Preincubation with Tris of isolated microsomal membranes caused a dose-dependent inhibition of Ca2+ binding in the absence of ATP at low Ca2+ concentration (20 microM). This inhibitory effect of Tris on Ca2+ binding was not observed at high Ca2+ concentration (1 mM). Preincubation of membranes with Tris also resulted in decreased Ca2+ accumulation in the presence of ATP. Both ionomycin-sensitive and -insensitive components of the ATP-supported Ca2+ accumulation were significantly lower in membrane fraction preincubated with Tris than these in the control membrane fraction not treated with Tris suggesting that both Ca2+ binding and active transport of Ca2+ were affected by Tris. This was further supported by the fact that oxalate-stimulated ATP-supported Ca2- accumulation by the membrane was potently inhibited by Tris. This inhibition was more prominent when the membranes were preincubated with Tris. Efflux of Ca2+ into EGTA containing medium from a preloaded membrane fraction was not significantly different whether or not the membrane fractions were preincubated with Tris indicating that the membrane permeability to Ca2+ may not be altered by Tris. Our results provided direct evidence that Tris altered the Ca2+ binding and transport at the subcellular membrane level and may provide a cellular basis for the alterations of contractile function of vas deferens in the physiological saline solution buffered with Tris.