OBJECTIVE: To investigate the interactive effects of oral contraceptive pill use and dietary fat intake on cardiovascular haemodynamics and metabolic parameters in young normotensive women. DESIGN: Thirty-two women participated, of whom 16 were taking oral contraceptive pills (ethinyl-oestradiol plus levonorgestrel) and 16 were age-matched and weight-matched controls not taking such pills. Subjects consumed either a high-fat or a low-fat diet for 2 weeks in an open, randomized, crossover study lasting 6 weeks. Investigations were performed at the end of each diet during the luteal phase of the menstrual cycle. METHODS: Blood pressure was measured by 24 h ambulatory recording; cardiovascular reactivity was determined by examining blood pressure responses to systemic infusions of noradrenaline and angiotensin II and to the cold pressor test; and carbohydrate metabolism was investigated by an intravenous glucose-tolerance test. RESULTS: Plasma triglyceride levels were significantly higher in women taking oral contraceptive pills compared with non-users on both diets; however, responses of lipoprotein levels to the two diets did not differ between study groups (total and low-density lipoprotein cholesterol levels decreased by 15 and 17% in oral contraceptive pill users and by 14% each in non-users, on the low-fat compared with the high-fat diet). Fasting plasma insulin levels, the insulin-production response to administration of glucose (insulin area under the curve) and resting clinic and night-time systolic blood pressures were all significantly reduced on the low-fat diet, but only in non-users. Blood pressure responses to noradrenaline and maximal heart rate response to cold were significantly attenuated during the low-fat diet in oral contraceptive pill users. During the low-fat diet, resting systolic, 24 h systolic and diastolic blood pressures and insulin area under the curve were all significantly higher for women taking the oral contraceptive pills. Users of these pills also exhibited a greater systolic sensitivity to administration both of noradrenaline and of angiotensin II and had a higher plasma renin activity irrespective of dietary phase. CONCLUSIONS: These results confirm that oral contraceptive pills have the potential to cause adverse effects on blood pressure, cardiovascular reactivity and the insulin-production response to administration of glucose and suggest that some of the beneficial effects of a low-fat diet on these parameters may be negated in women taking oral contraceptive pills.