cAMP and cGMP concentrations were measured in gastric mucosa in 18 anesthetized mongrel dogs receiving infusions of histamine (16 microgram/kg/hr), pentagastrin (6 microgram/kg/hr), bethanechol (10 microgram/kg/min), and theophylline (150 mg bolus, then 10 mg/min). The possible synergistic effect of continuous theophylline infusion with the secretagogues was studied in 15 other acute experiments. Antral and fundic mucosa were biopsied and measured by radioimmunoassay for cyclic nucleotides. All the secretagogues did not significantly change antral or fundic mucosal cyclic nucleotide concentrations compared to basal levels. In contrast, phosphodiesterase inhibition by theophylline significantly increased cAMP 11-fold and cGMP threefold in both antrum and fundus. Concomitant infusion of each secretagogue with theophylline did not potentiate the mucosal nucleotide response compared to theophylline alone. In a separate series of 19 experiments in three anesthetized dogs with vagally denervated fundic (Heidenhain) pouches, all the secretagogues which were infused in the same doses as in the acute biopsy studies produced maximal acid secretory responses. Theophylline, on the other hand, failed to stimulate acid secretion in five Heidenhain pouch experiments. These studies fail to implicate cyclic nucleotides as mediators of stimulated canine gastric acid secretion.