1. A randomized, controlled trial was carried out to examine whether changes in type and amount of dietary protein were responsible for earlier observations of blood-pressure-lowering effects of lacto-ovo-vegetarian diets. 2. Sixty-four subjects were pair-matched for sex, age, weight and sitting systolic blood pressure, and were randomly allocated to receive one of two types of protein supplement: one containing proteins from meat, the other proteins from non-meat sources. The supplements were balanced in terms of other nutrients. Consumption of other meat, poultry or fish was prohibited. 3. Sitting and standing blood pressures, weight, dietary intakes and plasma and urinary electrolytes were measured at regular intervals during the 12 weeks of trial. Urinary 3-methylhistidine was used as a measure of compliance. 4. Fifty subjects completed the trial. There were no statistically significant blood pressure differences between groups either at baseline or at end-of-trial, neither were there any substantive differences in mean blood pressure changes between baseline and end-of-trial. 3-Methyl-histidine excretion was significantly lower in subjects on the non-meat diet. 5. The results suggest that the protein components of the lacto-ovo-vegetarian diet are not responsible for the blood-pressure-lowering effects of that diet.