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Abstract Cyclic nucleotides alter intracellular free Mg2+ in renal epithelial cells. Am. J. Physiol. 262 (Renal Fluid Electrolyte Physiol. 31): FllOOF1104,1992.- Intracellular Mg”+ plays an important role in cell physiology. Studies were performed on MDCK cells and primary cortical thick ascending limb (CTAL) cells to determine hormonal influences on intracellular Mg2+ control. Free Mg2+ ( [Mg2+]i) was measured by fluorescence with mag-fura-2. Addition of 8bromoguanosine 3’,5’-cyclic monophosphate @-BrcGMP, 10e4 M) to subconfluent MDCK cells resulted in rapid increases in [Mg2+]i from basal levels of 552 it 6 PM to peak concentrations of 682 t 5 PM, whereas 8BrcAMP (10v4 M) led to significant decreases in [Mg2+]i from 538 of 5 to 362 t 17 PM. These effects of cyclic nucleotides were dose dependent with half-maximal concentrations (EC,,) of – 10e5 M for both increments in [Mg2+]i with cGMP and decrements in [Mg”+]i with CAMP. Atria1 natriuretic peptide (ANP) and cGMP increased Mg2+ in porcine primary CTAL cells from 525 t 12 to 592 & 18 PM and from 538 & 8 to 609 t 18 PM, respectively. The increment in [Mg2+]i with ANP was dose responsive with EC& values of – lo-l1 M suggesting that these effects may be of physiological importance. Parathyroid hormone and calcitonin and their second messenger, CAMP, diminished Mg2+ by -80 PM. The EC& value for calcitonin was in the order of 10mg M. The changes in [Mg2+]i, whether increases with ANP or cGMP and decreases with PTH, calcitonin, or CAMP, were rapid in nature and independent of changes in intracellular free Ca2+ concentration. These data indicate that [Mg”+]i is influenced by peptide hormones and their second messengers likely through activation of appropriate protein kinases. The functional role of these changes remains to be determined.