Agonist- and inositol 1,4,5-trisphosphate (InsP₃)-evoked responses in Xenopus oocytes utilize calcium mobilized from cellular stores as well as from the medium. We studied the effect of the status of Ca stores on InsP₃-induced Ca entry. Thapsigargin (TG) caused a net increase of ⁴⁵Ca²⁺ efflux from oocytes in a time and dose dependent manner (31 and 54% of total label, at 30 and 60 min, respectively). Incubation with TG (60 min) resulted in a complete loss of the response to InsP₃ implying that InsP₃-sensitive Ca stores were depleted. Challenge with 1.8 mM Ca²⁺ resulted in a large depolarizing chloride current (1231 ± 101 nA) which was not further potentiated by InsP₃. This suggested that extensive depletion of cellular Ca stores is sufficient to induce maximal entry of extracellular Ca (Ca_o). Following the injection of InsP₃, a much more limited loss of cellular Ca was sufficient to produce large Ca entry. Dimethyl sulfoxide (DMSO) alone, the vehicle used to dissolve TG, did not cause increase in either efflux of ⁴⁵Ca²⁺, nor in the Ca_o-evoked Cl^- current. It did, however, markedly potentiate this current following the injection of InsP₃. DMSO moderately inhibited InsP₃-induced ⁴⁵Ca²⁺ efflux from oocytes. Hence, apparent potentiation of Ca entry can be observed without additional depletion of cellular Ca. We conclude that Ca entry may be induced via either stimulation with InsP₃ and limited Ca depletion or depletion of a specific and, possibly small, cellular Ca store alone. The mechanism of DMSO potentiation is unknown, but may be important in view of the universal use of this solvent as vehicle.