Energy-dependent Ca2+ efflux and its regulation from the diazotrophic cyanobacterium Nostoc calcicola Breb has been investigated. Like Ca2+ uptake, Ca2+ efflux pattern also reflected a rapid phase for the first 10 min followed by a slower one lasting up to 1 h with a total of 80 nmol Ca2+ mg(-1) protein (31% of the Ca2+ concentration taken in by such cells at 1 h). Ca2+ efflux kinetics remained hyperbolic with a K-m of 1.9 mM and V-max 5.5 nmol mg(-1) protein min(-1). Ca2+ efflux to a major extent depended on photosynthetic energy generation as the cells facing dark incubation and addition of 3-(3,4-dichlorophenyl)-1-dimethyl urea (DCMU) to light-grown cells showed significant reduction in Ca2+ extrusion. The strong inhibition in Ca2+ efflux by addition of metabolic inhibitors like carbonyl cyanide-p-nitrofluoromethoxylphenyl hydrazone (FCCP) and N,N,-dicyclohexylcarbo-diimide (DCCD) suggested the vital role of membrane potential and ATP hydrolysis in driving this process. Verapamil (Ca2+ antagonist) had insignificant effect on Ca2+ efflux, whereas the addition of Calmodulin antagonists like trifluoroperazine, W-7 and compound 48/80 resulted in the enhancement in Ca2+ efflux over control sets, thus suggesting that this increase may be owing to the additional extrusion of intracellular free calcium that was unable to bind with calmodulin in the presence of these antagonists.