An algorithm is presented for the calculation of metal ion concentrations from given total metal concentrations (and vice versa) in physiological media containing metal-chelating compounds. In such media, conditions differ from those used for stability constant detennination of metal-chelator equilibria,· therefore calculated metal ion concentrations are incorrect. We recompute stability con-stants to reflect the effects of ionic strength and temperature of physiological solutions. Twelve different equilibria can be considered per metal-chelator pair. The computer program also calculates the contribution of ionized species of metals, chelator, complexes and pH buffers to ionic strength. Measurements with a Caselective electrode and withfura-2 show that calculated ionic Ca2+ concentrations are correct from 10 nM up to the millimolar range. The importance of the correct calculation of metal ion concentrations in physiological experiments is demonstrated by data, and derived kinetic parameters, on Na+/Ca2+ exchange and the ATP-dependent Ca2+ pump of enterocyte plasma membrane vesicles. The program is written in Turbo Pascal® and wil! run on IBM-compatible computers. It is menu-driven and supports the use of a Microsoft® mouse.