The purpose of this study was to determine whether lean (Fa/?) Zucker rats which exhibit greater metabolic efficiency in response to a high (48%) fat diet also exhibit hypertension.Twenty-nine lean (Fa/?) and eleven obese (fa/fa) Zucker rats were fed low (12% energy) and high (48% energy) fat diets. Food intake and weight change were recorded weekly to obtain an index of energy efficiency. After 6 wk, direct intra-arterial blood pressure was measured in conscious, unrestrained animals. Arterial blood was collected for analyses of plasma insulin and glucose. The high fat diet had no effect on blood pressure in obese rats, although obese animals fed both diets had significantly higher blood pressure than lean siblings. High fat feeding in lean rats unmasked a heterogeneity in blood pressure. Blood pressure in this group was significantly correlated with weight change, energy efficiency ratio and plasma glucose concentration. Lean rats fed the high fat diet with blood pressures above the group median exhibited significantly greater plasma insulin (P < 0.02) and glucose (P < 0.01) concentration and energy efficiency ratios (P < 0.01) than those with lower blood pressures. Thus, it appears that there were two groups of lean animals present, one of which was more phenotypically sensitive to high fat feeding than the other. This suggests a possible gene dosage effect of the fa allele in the Zucker rat model.