Changes of cell volume and intracellular calcium concentration ([Ca2+](i)) in immortalized thick ascending limb of Henle's loop (TALH) cells were monitored using confocal laser scanning microscopy and fura-2 fluorescence, respectively, Reduction of the extracellular osmolarity from 600 to 300 mosmol/l induced cell swelling followed by regulatory volume decrease (RVD). Simultaneously, the [Ca2+](i) increased transiently. The calcium rise was not observed in calcium-free solution or in the presence of nifedipine, indicating that the change was, in the first place, due to the activation of a calcium influx. Application of ATP or caffeine in isotonic solutions increased transiently the [Ca2+](i), which revealed the existence of stores in TALH cells sensitive to inositol-1,4,5 trisphosphate (IP3) and ryanodine. To examine the possibility that the calcium influx might induce calcium release, manganese quenching experiments were performed. In hypotonic calcium-free solutions, the decay of the calcium-insensitive and calcium- sensitive fluorescence occurred simultaneously. In the presence of extracellular calcium however, the calcium-sensitive wavelength revealed initial calcium influx followed by a calcium release from intracellular stores. Thus, the calcium influx was a prerequisite for the calcium release. We conclude that calcium-induced calcium release participates in global calcium signalling during RVD of TALH cells.