The uptake of Ca2+ and its regulation in the cyanobacterium Nostoc MAC were investigated. Cation uptake pattern was found to be biphasic, consisting of (a) rapid binding of cations to the negatively charged cell surface and (b) its metabolism dependent on intracellular import at least up to 60 min with the saturation at 2 mM Ca2+ (K-m, 1.5 mM, Vmax 42.1 nmol Ca2+ mg(-1) protein min(-1)). The cellular Ca2+ uptake was light and ATP dependent, and the addition of 3(3,4-dichlorophenyl)-1,1-dimethyl urea (DCMU) or exogenous ATP proved the vital role of PS II-generated energy to drive the process. The significant inhibition of Ca2+ uptake by different metabolic inhibitors and uncouplers like p-chloromercuribenzoate (pCMB), carbonylcyanide-p-nitrofluoromethoxylphenyl hydrazone (FCCP), N'N-dicyclohexylcarbodiimide (DCCD) and azide revealed that -SH group(s), proton gradient across the cell membrane, and ATP hydrolysis were involved in the transmembrane movement of Ca2+ in Nostoc MAC cells. Verapamil showed antagonism, abscisic acid (ABA) agonism, while trifluoroperazine (TFP) and N-(6-aminohexyl)-5-chloro-1-naphthalenesulfonamide (W-7) exerted no effect on Ca2+ uptake.