Bicarbonate/CO2 is believed to be the key in vitro effector of sperm capacitation, a process which induces major changes in the sperm plasma membrane in preparation for fertilization. In a flow cytometric study, we examined the effect of bicarbonate on boar spermatozoa using merocyanine, an impermeant lipophilic probe which binds to plasma membranes with increasing affinity as their lipid components become more disordered. We found that bicarbonate causes a rapid increase in the ability of live boar spermatozoa to bind merocyanine. First detected about 100 sec after exposure to bicarbonate and largely complete by 300 sec, this increase appears to result from individual cells within the sperm population switching from a low merocyanine-binding state to a high binding state. The majority of live spermatozoa are capable of responding in this way, and do so in proportion to bicarbonate concentration, half-maximal response being induced by about 3 mM bicarbonate; however, overall population response varies greatly between ejaculates. Increased merocyanine stainability is observed over the whole surface area of the cell, and is reversible both with respect to temperature (it is only manifested above 30 degrees C) and with respect to presence of bicarbonate. A similar effect can be induced by phosphodiesterase inhibitors such as isobutylmethylxanthine, and enhanced by a permeant cyclic nucleotide analogue. We conclude that bicarbonate causes a major alteration in sperm plasma membrane lipid architecture, apparently by perturbing enzymic control processes. This novel action of bicarbonate may represent an initial permissive event in the capacitation sequence.