The effect of acute normovolaemic haemodilution on microvascular red blood cell flow was studied by intravital microscopy in the tenuissimus muscle of the rabbit. Blood was substituted isovolaemically with equal volumes of a 6% solution of dextran 70 (MW 70,000). The systemic haematocrit (Hsys) decreased from 36 +/- 4% (mean +/- SD) to 17 +/- 2%. Following haemodilution capillary haematocrit (Hcap), as measured by video densitometric methods, decreased by 20 +/- 9%. The reduction of Hcap was significantly smaller than that of Hsys, and Hcap normalized with respect to Hsys increased from 0.39 +/- 0.07 in the control situation to 0.62 +/- 0.18 after haemodilution. Red blood cell velocity (vrbc) increased by 45 +/- 20% and compensated for the decrease in Hcap in such a way that the red blood cell flux, calculated from vrbc and Hcap, remained unchanged. Measurements of volume flow in the feeding arterioles in the muscle revealed a fractional redistribution of blood flow in favour of the muscle capillaries following haemodilution at the expense of vessels in adjacent connective tissue supplied by the same arterioles. This fractional flow redistribution was likely the basis for the relative increase in capillary haematocrit seen after haemodilution. The present data demonstrate that an acute reduction of the systemic haematocrit is compensated for in an active regulating vascular bed by a proportionally smaller decrease in capillary haematocrit and by an increased capillary red cell velocity. Microvascular haematocrit was found not to be a constant fraction of the systemic value, which supports the view of capillary haematocrit as a 'controlled' physiological variable.