Adenosine 3':5'-cyclic monophosphate (cAMP) and guanosine 3':5'-cyclic monophosphate (cGMP) were localized in cells of rat kidney cortex by an immunocytochemical technique before and after perfusion with parathyroid hormone (PTH). In control tissues the cAMP antiserum detected approximately the same intensity of fluorescence in cytoplasmic epithelial cell elements of cortical tubules and glomeruli (cells of Bowman's capsule and podocytes). PTH increased fluorescence in these glomerular cells and increased cAMP fluorescence in cytoplasmic granules in proximal tubular cells. These granules, whose structure has not been identified, were located predominantly on the luminal side of the tubular cells. In control rats, the renal cortical fluorescence detected with the cGMP antiserum was more pronounced in glomeruli (predominantly in the mesangial areas) and lesser amounts of fluorescence were observed in tubules. After PTH treatment, cGMP fluorescence increased in glomeruli and in renal tubular cells. A bright linear pattern of fluorescence was found in the area of the tubular luminal membrane. Perfusion with PTH caused relatively small increases in total tissue cAMP and no consistent increases in total tissue cGMP. Our observations suggest that both cAMP and cGMP are involved in the glomerular and tubular responses to PTH and point out the added dimension that this immunocytochemical technique brings to studies of cyclic nucleotide dynamics in heterogeneous tissues.