Eight volunteers received lignocaine 75 mg i.m. Peak plasma lignocaine concentrations (mean 0.80 microgram ml-1; range 0.31-1.86 micrograms ml-1) were attained at 30 min. Haematocrit and red cell deformability remained unchanged. Lignocaine caused small but significant decreases in plasma viscosity and whole blood viscosity at both high and low shear rates (94 and 0.94 s-1). The small reductions in plasma viscosity and high shear blood viscosity observed ex vivo may be the result of alterations in plasma proteins. The reductions in low shear blood viscosity are considered to result from decreased red cell aggregation.