Fifty-four healthy, omnivore normotensive volunteers aged 20-59 years were randomly allocated either to a control group eating a low polyunsaturated to saturated fat ratio (P/S ratio) diet (0.3) throughout, or to one of two experimental groups eating a high P/S ratio diet (1.0) for one of two 6-week experimental periods. Changes in other components were avoided. Twenty-four hour diet records showed substantial changes in the P/S ratio for experimental groups when on the high P/S ratio diet (0.3 to 1.0). There were significant increases in relative concentrations of linoleic acid in plasma phospholipids when on the high P/S ratio diet. There was no consistent effect of dietary P/S ratio elevation from 0.3 to 1.0 on group mean systolic and diastolic blood pressures. Adjustment of blood pressure changes for changes in electrolytes, other dietary components, plasma lipids, weight and other lifestyle variables did not alter this result. It was concluded that the blood pressure lowering effect of a vegetarian diet reported previously was unlikely to have been due to changes in dietary P/S ratio.