A normotensive (1.0 microgram per 100 g b.w.) or hypotensive (10.0 micrograms per 100 g b.w.) dose of serotonin (5-HT) was administered endoportally while changes in microcirculation at the inlet and outlet regions of hepatic lobules were measured on-line using quantitative in vivo microscopy. The number of sinusoids with decreased (cellular) flow also was counted to index intralobular perfusion in video recordings of microvasculature examined off-line. The normotensive and hypotensive doses of 5-HT elicited decreases in intralobular perfusion within periportal and centrivenous sinusoids. Hypoperfusion was accompanied by a transient decrease in volumetric flowrate (Q) at the outlet of centrivenous sinusoids in 40% of normotensive and in 100% of hypotensive rats. At the inlet of periportal sinusoids, Q was depressed in 75% of hypotensive and in 27% of normotensive rats. The remainder of these segments had either an increase or no change in Q at the inlet and outlet. These results suggested that during conditions of 5-HT induced (lobular) hypoperfusion: (a) Q at the inlet is maintained in 73% of normotensive rats by redistribution of intralobular blood flow, and decreased in all but 25% of hypotensive rats as a function of transient reductions in total hepatic (arterial) and/or portal (venous) blood flow(s), and (b) Q at the outlet is depressed in 40% of normotensive rats by apparent increases in flow redistribution and resistance to flow generated during sinusoidal constriction, whereas in all hypotensive rats this mechanism is aggravated by decreased total hepatic (arterial) and/or portal (venous) inflow(s). Therefore, although the initial time course for microvascular responses tended to be similar for normo- and hypo-tensive doses of 5-HT, quantitative differences in regional flow distribution and Q emphasize (a) the importance of intra- and extra-hepatic determinants in the regulation of blood flow within hepatic (unit) lobules, and (b) the presence of microvascular heterogeneity within these lobular units.