Data from cultured cells have suggested that cyclic AMP and cyclic GMP may be important determinants of cell growth and transformation. However, few studies have examined cyclic nucleotide content and metabolism in naturally occurring tumors of man. Accordingly, in the present study we compared cAMP and cGMP levels and metabolism in carcinomas of the human colon to those of the adjacent uninvolved mucosa after therapeutic resection of these tissues. The cAMP content of the tumors, determined in samples frozen 30 min after excision, was significantly lower than that of the adjacent mucosa, when expressed on the basis of tissue wet weight, protein, or DNA content. By contrast, the cGMP content of the tumors was higher than that of the surrounding mucosa if calculated on the basis of tissue wet weight, but this difference did not persist when correction was made for the higher protein or DNA content of the tumors. Incubation of slices of mucosa or tumor with or without theophylline in vitro increased tissue cAMP and cGMP content above levels observed in frozen samples of the same tissue. However, after such incubations cAMP levels in the tumors remained clearly below that of the mucosa, while cGMP content of the two tissues did not differ. The failure of theophylline to abolish differences in cAMP content and the comparable activities of high and low Km cAMP-phosphodiesterase in homogenates of the two tissues suggested that the lower cAMP content of the tumors was a consequence of diminished cAMP synthesis rather than enhanced degradation. This possibility was supported by the reduction in basal and maximal prostaglandin E1 (PGE1)-responsive adenylate cyclase activity found in tumor homogenates relative to those of mucosa, and the lower levels of cAMP in tumor slices after incubation of the tissues with a maximal dose of PGE1 and theophylline. Since NaF-responsive adenylate cyclase activity was not significantly reduced in the tumors, the lower basal and PGE1 activities may not be related to a deficiency of the catalytic unit of the cyclase complex in this tissue. The role of reduced activity of the adenylate cyclase-cAMP system and/or reduced tissue cAMP-to-cGMP ratios in the pathogenesis of colonic carcinoma is uncertain, but these changes might favor unregulated cellular proliferation.