Medium term effects (3 months) on serum lipoprotein levels of a diet with a high P/S ratio (2.2) and a low P/S ratio (0.3) were investigated in 13 normolipoproteinemic rhesus monkeys. Both diets were studied with and without added gel-forming fibre, as pectin. Addition of pectin did not have major influences on serum lipid levels. Changing from a low to a high P/S diet resulted in a significant decrease of total cholesterol (23%) and LDL cholesterol (18%) levels, but also HDL cholesterol (23%) and apolipoprotein A1 (Apo A1; 13%) concentrations fell significantly. However, after 12 weeks on the high P/S diet, HDL cholesterol concentrations rose by 24%, not significantly different from the levels during the low P/S diet. We conclude that the medium-term effects of a high P/S diet are a decrease in LDL cholesterol with only a transient effect on HDL cholesterol.