Capillary red blood cell (RBC) flow in the skin of rat tail during an extreme hemodilution with albumin was microscopically observed with a completely noninvasive transcutaneous technique using flash epi-illumination. This illumination with a filter (center wavelength, 550 nm) caused RBC to become darker and contrast well with the background tissue since the light was absorbed by hemoglobin inside the RBC. The recorded videotape showed a clear image of RBC without tailing because of flash (strobe light) illumination, and this allowed accurate analysis of the RBC flow rate and capillary hematocrit. During hemodilution up to a 50% level of blood exchange, the functional capillary density remained unchanged and it started to decline significantly at the 60% level of blood exchange and reached 37 +/- 32% of the basal value at the 80% level of blood exchange. The RBC flow rates monotonously increased up to 294 +/- 110% of the basal value at the 60% level of blood exchange, which was in contrast to the abdominal aortic blood flow increase to 148 +/- 20%. Capillary hematocrit did not appear to fall in the same proportion as the systemic hematocrit, which coincided with the previous observations. These results indicate that microhemodynamic properties during hemodilution can be transcutaneously analyzed with this new technique without surgical preparation of the skin.