The effects of cholera enterotoxin on dog thyroid cyclic adenosine 3',5'–monophosphate (cyclic AMP) levels and intermediary metabolism were studied and compared to those of thyrotropin (TSH) and E. coli enterotoxin. Although TSH stimulates cyclic AMP and 14CO2 formation within minutes, both crude and purified cholera enterotoxin (choleragen) required 25 min to stimulate cyclic AMP and 45 min to increase l–14C—glucose oxidation. E. coli enterotoxin stimulated 14CO2 formation during a 45–min incubation without producing any change in cyclic AMP. However, E. coli enterotoxin did increase cyclic AMP levels during a two—hr incubation. Choleragen and cholera enterotoxin increased adenylate cyclase activity in homogenates prepared from thyroid slices which had been previously incubated with the enterotoxins. Unlike TSH, they had no effect when added directly to the homogenate. The effect of choleragen on cyclic AMP formation occurred in the absence of, but was potentiated by, 10-2M theophylline. Despite the delay in stimulation of cyclic AMP levels, cholera enterotoxin appeared to bind rapidly to thyroid tissue. Choleragen and cholera enterotoxin were bound to thyroid slices during a 20–min incubation at 4 C, and their subsequent effects on cyclic AMP, unlike those of TSH, could be inhibited in a dose dependent manner by addition of horse anticholeragenoid serum. This inhibitory effect did not occur when the initial 20–min incubation was done at 37 C. TSH and choleragen stimulated endocytosis within 1 hour whereas E. coli enterotoxin required 4 hr. Choleragen and E. coli enterotoxin had no effect on 32P incorporation into phospholipid in thyroid slices while crude cholera enterotoxin caused inhibition. Low doses of choleragen (1.25 μg/ml) and cholera enterotoxin (1 mg/ml) enhanced organification of iodide, whereas larger doses and E. coli enterotoxin inhibited. It is concluded that choleragen and cholera enterotoxin, like TSH, bind rapidly to the thyroid and stimulate cyclic AMP formation as a consequence of adenylate cyclase activation. However, the mechanism of stimulation and some of the consequent metabolic effects appear to differ. (Endocrinology92: 541, 1973)