Antibodies to the cyclic nucleotides initially were utilized in radioimmunoassays for cyclic AMP and cyclic GMP which might be present in mammalian tissues. allowed measurement of the nucleotides on small amounts of tissue in physiologic studies. To gain further insight into the relative roles of cyclic AMP and cyclic GMP in cell function, these antibodies have been applied to immunohistochemical studies for the localization of the cyclic nucleotides in tissues and cells. This methodology is useful for determining in which cell type in a heterogeneous tissue increases in cyclic nucleotide concentrations occur. In addition, within individual cells, staining patterns for cyclic AMP and cyclic GMP are usually quite distinct. Cyclic GMP in canine thyroid is located to the follicular cell membrane while cyclic AMP is ubiquitously distributed in follicular cell cytoplasm. In both rat adrenal cortex and testis, there is prominent nuclear localization of cyclic GMP, suggesting a role for the nucleotide in growth regulation. These studies provide histologic evidence suggesting diverse roles for cyclic AMP and cyclic GMP in mammalian physiology. It is anticipated that this technique will also be useful in the ultrastructural localization of the cyclic nucleotides and for the identification of other cyclic nucleotides which might be present in mammalian tissues.