Since we have observed that monounsaturated fatty acids (MUFA) enriched diet modifies red cell membrane lipids and cation transport systems in normotensive subjects, we similarly evaluated a group of hypertensive patients undergoing an analogous dietary modification. In a group of 18 moderately hypertensive women, the diet was supplemented for two months with olive oil (about 45 g/day), which replaced an equal amount of seasoning fats. Before and after this period, red cell fatty acid composition was evaluated by gas-chromatography in order to verify diet compliance: a significant increase in oleic acid was observed, while the content of saturated and polyunsaturated fatty acids remained unchanged. After olive oil, maximal rates of Na-K pump (5580 +/- 329 vs 6995 +/- 390, p less than 0.001) and Na-K cotransport (Na-COT 544 +/- 52 vs 877 +/- 46, p less than 0.001: K-COT 790 +/- 76 vs 1176 +/- 66, p less than 0.001), cell Na content (9.58 +/- 0.4 vs 10.61 +/- 0.6, p less than 0.03) and passive permeability for Na (936 +/- 74 vs 1836 +/- 102, p less than 0.001) rose significantly. Although the reduction in maximal rate of the Li-Na CT after olive oil was not significant, it was the only cation transport parameter being correlated with the variations of membrane lipids, namely negatively with UFA (r = -0.528, p less than 0.05) and positively with SFA (r = 0.482, p less than 0.005). The change in maximal rate of Li-Na CT was also correlated with the variation of systolic and diastolic BP (r = 0.50, p less than 0.03).(ABSTRACT TRUNCATED AT 250 WORDS)