The effect of pentagastrin on mucosal microcirculation was studied in rats by use of intravital microscopy. The superficial mucosal vessels were videorecorded for off-line analysis of red cell velocities (VRBC) and vessel diameters, from which blood flow (QRBC) was calculated. Resting mucosal blood flow calculated from single microvascular flow data, and vessel distribution was 40 ml X min-1 X 100 g-1. Pentagastrin infused intravenously in a dose of 20 micrograms X kg-1 X h-1 resulted in submaximal acid secretion (approximately 60%) and a significant increase in QRBC by 47 +/- 14%. When given in a dose of 96 micrograms X kg-1 X h-1 iv, it resulted in maximal acid secretion and an increase in QRBC by 36 +/- 14%. In another series of experiments the results of QRBC measurements during infusion of pentagastrin (20 micrograms X kg-1 X h-1 iv) were compared with those of aminopyrine (AP) clearance or laser-Doppler flowmetry (LDF) in the same animals. Gastric mucosal blood flow determined by [14C]AP clearance increased by 309 +/- 115%, whereas QRBC increased by 34 +/- 11%. When determined by LDF, blood flow increased by 41 +/- 22%, a value similar to the increase in QRBC (50 +/- 19%). Thus, the percent increase in blood flow during pentagastrin infusion estimated by AP clearance was considerably higher than that observed by either direct microvascular measurements or by LDF.