Ca2+ causes less than 2-fold elevations of guinea pig sperm cyclic AMP concentrations when cells are incubated in a minimal culture medium in the absence of bicarbonate (HCO3-). However, in the presence of HCO3-, Ca2+ increases cyclic AMP by as much as 25-fold within 1 min. The (Ca2+, HCO3-)-induced elevations occur in either the presence or absence of the permeant anions, pyruvate and lactate. In the absence of extracellular Ca2+, HCO3- elevates cyclic AMP only slightly. The effect of HCO3- is concentration-dependent, with maximal responses obtained at concentrations of greater than 25 mM. Ca2+ (25 mM HCO3-) at concentrations of less than 100 microM causes one-half-maximal elevations of cyclic AMP. The (Ca2+, HCO3-)-induced elevations of cyclic AMP are observed at various extracellular pH values (7.5-8.5) and in the presence or absence of extracellular Na+ or K+. NH4Cl does not elevate sperm cyclic AMP concentrations and does not greatly alter the (Ca2+, HCO3-)-induced elevations. the putative Ca2+ transport antagonist, D-600 (100 microM), completely blocks the (Ca2+, HCO3-)-induced elevations of cyclic AMP. A23187, in the presence but not in the absence of extracellular Ca2+, increases sperm cyclic AMP but does not further elevate cyclic AMP in HCO3(-)-treated cells. These studies establish that Ca2+-dependent elevations of cyclic AMp in guinea pig spermatozoa are dependent on the presence of HCO3- and suggest that HCO3- is required for the uptake (exchange) or membrane sequestration of small amounts of physiologically active Ca2+.