The uptake of Hg2+ and its regulation in the cyanobacteriumNostoc calcicola Breb. was studied. Hg2+ uptake pattern consisted of two distinct phases: (a) rapid binding of the cation to the negatively charged cell surface (first 10 min) and (b) its subsequent metabolism-dependent intracellular import, at least up to 40 min (saturating concentration 1.5 μM Hg2+, Km=1.0μM Hg2+ and Vmax 0.21 nmol Hg2+ mg−1 protein min−1). Hg2+ influx, to a major extent, depended on photosynthetically generated energy, and the supply of exogenous ATP (10 μM) or DCMU (5 μM) suggested the vital role of PS II-mediated energy to support the process. The significant lowering in Hg2+ uptake rate as well as total cellular Hg2+ in the presence ofp-chloromercuribenzoate (pCMB), azide (NaN3), N,N′-dicyclohexycarbodiimide (DCCD), and thiol (mercaptoethanol) indicated the role of membrane potential,-SH groups, and ATP hydrolysis in regulating Hg2+ transport. While Cu2+ antagonized Hg2+ intake, Ni2+ showed synergism.