We have investigated how the immunosuppressant drug FK506 affected the basal Ca 2+ leak in permeabilized A7r5 cells. Non-mitochondrial Ca 2+ stores loaded to steady state with Ca 2+ slowly lost their accumulated Ca 2+ during incubation in a Ca 2+-free efflux medium. FK506 up to 100 μM had no effect on the basal Ca 2+ leak. In contrast, the rate of Ca 2+ release proceeded much faster immediately after washing out FK506. The increase in rate of Ca 2+ release after washing out of this compound depended on both its initial concentration and on the time of pre-incubation. A similar effect was also observed after removing another immunosuppressant drug (rapamycin) and after removing the inositol 1,4,5-trisphosphate receptor inhibitor xestospongin C. Since all these substances have a high octanol/H 2O partition coefficient and accumulate in the endoplasmic reticulum membrane, we suggest that the transient increase in the basal Ca 2+ leak is due to the sudden removal of these lipophilic substances from the membrane.