The effect of the guanine nucleotide GTP on Ca2+ release from the endoplasmic reticulum of digitonin-permeabilized islets was investigated. Maximal and half-maximal Ca2+ release were observed at 5 .mu.M- and 2.5 .mu.M-GTP respectively. GTP caused a rapid release of Ca2+ from the endoplasmic reticulum, which was complete within 1 min. GTP-induced Ca2+ release was structurally specific and required the hydrolysis of GTP. The combination of maximal concentrations of GTP (10 .mu.M) and myo-inositol 1,4,5-trisphosphate (IP3) (10 .mu.M) resulted in an additive effect on Ca2+ release from the endoplasmic reticulum. GDP (100 .mu.M), which inhibits GTP-induced Ca2+ release, did not affect IP3-induced Ca2+-release. Furthermore, GTP-induced Ca2+ release was not independent on submicromolar free Ca2+ concentrations, unlike IP3-induced Ca2+ release. These observations suggest that mechanistically GTP-induced Ca2+ release is different from IP3-induced Ca2+ release from the endoplasmic reticulum.