Several cholinergic agonists and their antagonists were administered topically at various concentrations (10(-10) to 10(-4) gm per ml) to the livers of anesthetized Sprague-Dawley rats. Changes in the microvasculature were measured for a period of 15 min using in vivo microscopic methods. The influence of cholinergic agonists on hepatic mast cells was determined by histochemical methods. Cholinergic agonists elicited constriction of portal venules, sinusoids, and terminal hepatic (central) venules accompanied within 5 to 20 sec by the adherence of platelets and leukocytes to the endothelial lining of these vessels. These responses were not observed in hepatic arterioles or hepatic (sublobular) venules. However, the responses drastically altered cellular flow through all segments of the microvasculature. The drugs were effective in the following ranking: bethanechol greater than pilocarpine greater than carbachol. Vascular responses were not antagonized by atropine; phentolamine modified the constrictor response of portal venules but of no other microvascular segment. These results suggest that the responses were not mediated by cholinergic (muscarinic) receptors and that the constriction of portal venules was caused by the chemical activation of alpha-adrenergic receptors. Histochemical methods revealed that pilocarpine and bethanechol also elicited a significant decrease in the number of visible mast cells suggestive of degranulation and the release of serotonin, histamine, and other polyanions (e.g., heparin) from these cells. Release occurred within the same period of time as the microvascular alterations (15 min.). These data provide additional evidence that mast cell constituents and adrenergic mechanisms are involved in hepatic cholinergic mechanisms.