The influence of a fat-rich test meal on postprandial changes in plasma viscosity and serum viscosity was assessed in 12 normolipidemic adults. After a 12-14-h fast, volunteers (five men and seven women aged 23-50 y) were challenged with a test milk shake containing 50 g fat/m2 body surface area (BSA). Plasma viscosity, serum viscosity, and plasma lipids and lipoproteins were assessed at 0, 2, 3, 4, and 6 h. Viscosity values were determined by using a Mettler Contraves LS-40 rotational microviscometer. Postprandial changes in the study variables were assessed by area under the curve and included triacylglycerols (2.02 mmol/L), plasma viscosity (-0.10 mPa.s), and serum viscosity (-0.01 mPa.s). Peak plasma triacylglycerol concentrations were significantly greater than those observed at baseline (P = 0.0022). There were no significant changes in any other variable when fasting and peak values were compared. Peak plasma viscosity increased in three and decreased in two subjects with no changes in the remaining seven subjects. Changes in peak plasma viscosity ranged from -7% to 7% with similar changes for serum viscosity, from -8% to 10%, and a slightly greater range for plasma fibrinogen, -16% to 10%. In this cohort of normotriacylglycerolemic subjects, there were no significant postprandial changes in plasma viscosity or serum viscosity.