Growth hormone- and prolactin secreting GH4/C1 pituitary tumor cells respond to hypotonicity with cell swelling followed by a regulatory volume decrease (RVD), as assessed by video imaging. The RVD was inhibited by the anion channel blocker DIDS. Whole cell patch clamp experiments showed that cell swelling in 30% hypotonic solution was paralleled by the activation of an outwardly rectifying, DIDS-sensitive chloride current, which - in contrast to RVDC measured in many other cell types - time-dependently activated at positive holding potentials. Virtually the same current could be elicited under isotonic conditions by stimulation of cellular calcium entry with the calcium ionophore ionomycin in GH4/C1 cells or NIH 3T3 fibroblasts. The RVDC in GH4/C1 cells therefore resembles calcium-activated chloride currents. From the present experiments we can, however, not exclude the contribution of 'typical' swelling-activated chloride currents to the observed phenotype (in two experiments we observed RVDC with time-dependent inactivation at positive potentials). Beside their obvious role in cell volume regulation, RVDC might be important for the regulation of hormone secretion in GH4/C1 cells as discussed for other hormone secreting cells including pituitary cells and pancreatic beta-cells.