Ninety-four volunteers participated in a three-day weighed dietary survey and had their blood pressures measured once. They were classified according to their fibre intake. Subjects with a high-fibre intake were found to have lower mean blood pressures than those with a low-fibre intake. Forty-two of the volunteers further participated in a group of experiments. Those eating a high-fibre diet decreased their dietary fibre, and those eating a low-fibre diet increased their dietary fibre. The first group (11 subjects) showed increased mean blood pressures after four weeks of eating the experimental diet. The last group (31 subjects) showed decreased mean blood pressures after four weeks of eating the experimental diet. The 11 subjects consuming the low-fibre experimental diet showed a decrease in mean blood pressure when some of the saturated fat in their diet was replaced by polyunsaturated fat. A similar substitution carried out by 14 of the subjects consuming the high-fibre experimental diet also resulted in decreased mean blood pressure, but this was not statistically significant. Twelve more volunteers, with hypertension, were all found to have low-fibre diets. They consumed a high-fibre diet for a six-week experimental period, but their mean blood pressures did not decrease significantly; individual recordings varied substantially during this period.It is suggested that differences in the type and quantity of dietary fibre and fat may be responsible for the lower mean blood pressures of groups of vegetarians compared with similar groups of non-vegetarians.