We have studied the effects of diet-induced hypercholesterolemia on the rates of secretion of triglycerides into the plasma of fasted squirrel monkeys. Two groups of monkeys were studied: control animals which were fed a semipurified diet not associated with hyperlipemia (plasma cholesterol 127 +/- 8 mg/100 ml), and animals made hypercholesterolemic (plasma cholesterol 307 +/- 31 mg/100 ml) by being fed a diet containing 25% butter and 0.5% cholesterol. After intravenous infusion of Triton WR 1339 (300 mg/kg body wt), plasma triglycerides increased almost linearly for 9-12 hours. Analysis of individual lipoproteins separated by ultracentrifugation showed that newly secreted triglycerides were present almost exclusively in the very low density lipoprotein fraction. The rates of triglyceride secretion in the hypercholesterolemic group of monkeys (5.15 +/- 0.86 mg/kg/hr) were less than half those of the control animals (10.96 +/- 2.15 mg/kg/hr). We suggest that in monkeys with diet-induced hypercholesterolemia high concentrations of plasma low density lipoproteins may inhibit the synthesis and/or secretion of their parent very low density lipoprotein molecules into the circulation.