We examined how acupuncture affected the blood flow of muscle, kidney, stomach, small intestine, brain, lung, heart, spleen and liver. Wistar rats anesthetized with urethane (n = 27) were allocated into the control (n = 10), ST-7 (Hsia-Kuan, n = 10) and LI-4 (Hoku, n = 7) groups. To measure organ blood flow, colored microspheres (CMS) were injected through a catheter positioned in the left ventricle and blood samples were drawn from the femoral artery. Yellow CMS (3.6-4.2 x 10(5)) and blue CMS (6.0-6.9 x 10(5)) were injected at intervals of about 30 min. An acupuncture needle (varphi 340 mum) was inserted into the left ST-7 point (left masseter muscle) or the right LI-4 point after the first sampling and left for about 30 min (10 twists at 1 Hz, 2-min intervals). The mean blood flow of nine organs varied widely from 4.03 to 0.20 (ml/min/g). Acupuncture to the ST-7 produced significant changes of the blood flow (percentage change from baseline) in the muscle, kidney, brain and heart (P < 0.05, versus control), but those of LI-4 were not significant. The blood flow of the left masseter muscle after acupuncture to ST-7 (left masseter muscle) tended to increase (P = 0.08). Changes in blood pressure during the experimental periods were almost similar among these three groups. Acupuncture stimulation increases the blood flow of several organs by modulating the central circulatory systems, and the effects differed with sites of stimulation.