The second messenger, cyclic guanosine monophosphate (cGMP), mediates the actions of nitric oxide, natriuretic peptides, and microbial toxins on cellular contractility and electrolyte movement. Because both hepatocellular contractility and electrolyte secretion participate in bile formation, we investigated the actions of cGMP on this process in intact liver. In rat liver perfused with 8-bromo-cyclic GMP (bcGMP) at 0.5 and 3 micromol/min, bile flow increased by 5% and 31%, respectively. The biliary excretion of the bile acid, taurocholate ([3H]-labeled; 1 micromol/min) and of the organic anion, bromosulfophthalein ([35S]-labeled; tracer dose), was unchanged. The paracellular and transcytotic pathways of biliary excretion, assessed by horseradish peroxidase (HRP), were unaffected. BcGMP was concentratively secreted into bile and the accompanying 30% increase in the biliary clearance of erythritol suggested that the choleresis was primarily osmotic in nature. Unlike cyclic adenosine monophosphate (cAMP), which stimulates bile acid dependent bile flow and transcytosis, bcGMP increased bile acid independent bile flow mainly as a result of its concentrative biliary secretion.