Because an increase in the HCO₃^- concentration of oviductal liquid at mid-cycle is believed markedly to enhance fertility, we have studied active secretion of HCO₃^- across highly differentiated cultures of monkey oviductal epithelium. Cultured cell sheets were mounted in Ussing chambers, and bathed in medium containing 25 mM HCO₃^-. Purinergic agents potently stimulated short-circuit current (I_sc) with an initial transient response declining within about 2-min to a sustained response. The potency sequence of ATP UTP > ADP >> AMP suggested that the Isc response was mediated mainly by P2Y2 receptors. Acetazolamide, an inhibitor of carbonic anhydrase, had little or no effect on baseline Isc or the transient response to ATP, but abolished the sustained response to ATP. Similar results were obtained on sheets of native epithelium. In pH-stat experiments, the abluminal medium of cell cultures was bathed in HCO₃-/CO₂ medium, and the pH of the unbuffered luminal medium was maintained at ~7.4 by addition of strong acid or base. ATP stimulated base secretion, and this was inhibited by acetazolamide. Further, these changes in secretion of base were in good quantitative agreement with the I_sc responses. When phenol red (an estrogen) was removed from the culture medium, ATP-dependent HCO₃^- secretion was markedly reduced, but could be restored by treatment with estradiol. Estrogens also markedly increased ciliation of the cultures. These results suggest that the mid-cycle increase in the HCO₃^- concentration of oviductal liquid may be mediated by the effects of estradiol on purinergic pathways or on ATP secretion.