Both ergotamine and dihydroergotamine enhanced guanylate cyclase (E.C.4.6.1.2) activity two- to threefold in rat aorta, heart, liver and kidney. Dose-response relationships revealed that ergotamine and dihydroergotamine caused a maximal stimulation of hepatic guanylate cyclase activity at the 0.1 microM concentration. There was no further augmentation with increasing the concentration of either ergot alkaloid to the millimolar range. No enhancement of guanylate cyclase activity was noted with decreasing the concentration of ergotamine and dihydroergotamine to 0.001 microM. At all concentrations, ergotamine was a more potent stimulator of guanylate cyclase activity than was dihydroergotamine. Both ergotamine and dihydroergotamine also increased tissue cyclic GMP levels at the same concentrations as above. Varying the concentration of the guanylate cyclase co-factor manganese had no effect on these ergot alkaloids' augmentation of guanylate cyclase activity except that with no manganese present the same amount of maximal activation could not be achieved. Since bromocriptine, another ergot alkaloid, also enhances guanylate cyclase activity, it appears that ergot alkaloids have a common mechanism of action which involves the guanylate cyclase-cyclic GMP system at the cellular level.