Pentoxifylline (PF), known to increase red blood cell (RBC) deformability, has recently attracted much attention because of its possible effect of reducing capillary resistance. There has been practically no reliable demonstration of an increase in capillary blood flow by this drug. This study was intended to examine the effect of PF and hence deformability changes on microvascular hemodynamics and the rheological mechanisms underlying such an effect. Mesentery microvessels of 15 rats were subjected to intravital microscopic observation under transillumination. Capillary blood flow was measured using a newly developed 2-channel sample-hold scanner incorporated into a video display system. Changes in whole blood viscosity were also measured in order to investigate hemorheological changes. It was found that a significant increase in capillary blood flow (as much as 140%) was brought about by PF administration. The increase in blood flow was particularly remarkable in capillaries of about 13 microns in diameter, while whole blood viscosity measured in vitro showed a reduction of 40% at most. In view of these results and the reported effect of PF on increasing the RBC deformability, increased capillary blood flow can be attributed to the increased RBC deformability.