The effect of a cholinergic agonist and antagonist on finger blood flow (FBF) was studied in 10 normal subjects. Total finger blood flow was measured by venous occlusion, air plethysmography, and capillary blood flow (FCF) by the disappearance rate of a radioisotope from a fingertip injection. Methacholine in doses of 10-80 micrograms/min was given by constant infusion via a brachial artery catheter. Average FBF (7.4 +/- 2.5 to 10.9 +/- 4.3 ml X min-1 X 100 ml tissue-1) and vascular resistance (22.4 +/- 4.7 to 21.3 +/- 7.0 mmHg X ml-1 X min X 100 ml tissue) were not significantly affected. However, the half time (t1/2) of the disappearance rate decreased from 50.8 +/- 13.4 to 11.1 +/- 1.5 min (P less than 0.01); a decrease occurred in all subjects. In seven subjects, atropine (0.2 mg) had no affect alone but inhibited the effect of methacholine on FCF (t1/2 47.6 +/- 6.2 to 50.2 +/- 6.7 min) and prevented the redness and sweating of the forearm and hand that occurs with this agent. This study demonstrates a muscarinic cholinergic vasodilator mechanism in the fingertip that uniquely increases capillary blood flow.