1. Patch clamp and fura-2 AM measurements were performed to study the effects of sensitizing the inositol 1,4,5-trisphosphate (InsP3) receptor to InsP3 on the activation of Ca2+ release-activated Ca2+ current (ICRAC) in rat basophilic leukaemia (RBL) cells. 2. The sensitizing agent thimerosal (1 microM) triggered Ca2+ release, and this was followed by Ca2+ influx. With no added InsP3 in the patch pipette, thimerosal activated ICRAC; this was prevented by heparin. ICRAC activated by thimerosal was very similar to that evoked by InsP3 or ionomycin. 3. Dialysing cells either for short (30 s) or long (600 s) periods of time prior to application of thimerosal did not affect the subsequent activation of ICRAC, even though no InsP3 was included in the patch pipette. 4. These results suggest that sensitizing the InsP3 receptor can result in large Ca2+ influx in the presence of resting InsP3, and that stores closer to the membrane may contribute more to activation of ICRAC than stores further away.