The hypocholesterolemic effect of polyunsaturated fatty acids was compared in male rats given high-cholesterol diets containing either evening primrose oil (EPO, linoleic plus gamma-linolenic), safflower oil (SFO, linoleic) or olive oil (OLO, low linoleic) at the 10% level. EPO with a phytosterol content of 1.47% was found to be more hypocholesterolemic than SFO (phytosterols 0.34%), and rats given EPO excreted more neutral (cholesterol and its metabolites) but not acidic steroids during the first 2 weeks of the feeding. Even when the phytosterol content of EPO and SFO was adjusted to be the same (0.67%), EPO was still more hypocholesterolemic than SFO but to a lesser extent, although fecal neutral steroid excretion was comparable in these two dietary fat regimens. The results indicate a significant hypocholesterolemic efficacy of gamma-linolenic acid.