The involvement of cell volume in the K⁺-evoked Ca²⁺ signaling was studied in cultured rat glomerulosa cells. Previously we reported that hyposmosis (250 mOsm) increased the amplitude of T-type Ca²⁺ current and, accordingly, enhanced the Ca²⁺ response of cultured rat glomerulosa cells to K⁺ (Makara et al. : Endocrinology 141:1705-1710, 2000.) In the present study we found that this enhancement is not influenced by the cytoskeleton-disrupting drugs cytochalasin-D (20 µM) and colchicine (100 µM). Elevation of extracellular potassium concentration ([K⁺]_e) from 3.6 mM to 4.6-8.6 mM induced cell swelling, which had slower kinetics than the Ca²⁺ signal. Cytoplasmic Ca²⁺ signal measured in single glomerulosa cells in response to stimulation with 5 mM K⁺ for 2 min showed two phases: after a rapid rise reaching a plateau within 20-30 sec, [Ca²⁺]_c increased further slowly by about one third. When 5 mM K⁺ was co-applied with elevation of extracellular osmolarity from 290 to 320 mOsm, the second phase was prevented. These results indicate that cell swelling evoked by physiological elevation of [K⁺]_e may contribute to the generation of sustained Ca²⁺ signals by enhancing voltage-activated Ca²⁺ influx.