The role of calcineurin in modulation of calcium channel activity was examined in GH3 pituitary cells by using its selective inhibitor cyclosporin A. While cyclosporin A had little effect on basal activity, it induced a biphasic increase in K⁺-induced⁴⁵Ca²⁺influx. Cyclosporin A rapidly increased K⁺-induced⁴⁵Ca²⁺influx to approximately 140% of control in 1 h and the increment maintained at this magnitude for 1-8 h. Thereafter, K⁺-induced⁴⁵Ca²⁺influx gradually increased further to approximately 220% after 24 h exposure to this compound. In the presence of anisomycin, however, the increase occurred at the latter phase was abolished. In addition, the increased calcium influx in cyclosporin A-pretreated cells had a similar sensitivity to KCl and verapamil as in untreated cells. Measurement of intracellular Ca²⁺level by Fura-2 analysis indicated that [Ca²⁺]i increase induced by high K⁺or vasoactive intestinal peptide was similarly augmented in cyclosporin A-pretreated cells. Thus the results of this study suggest that calcineurin may play a tonic control on L-type Ca²⁺channel, and inhibition of this enzyme may induce a subsequently protein synthesis-dependent higher channel activity.