In two 3-week experiments with a 2-factorial design, weanling rats were fed semi-purified diets in which the zinc concentration was either suboptimal or adequate for maximum growth. In experiment 1, one low-fat diet and four diets enriched with 26% beef tallow (BT), butterfat (BF), coconut oil or sunflower oil (SF) were compared. The low-Zn SF diet did not support growth. The animals fed the high-Zn SF diet had the lowest final weights among all other groups, and their serum and femur Zn concentrations were markedly depressed. In experiment 2, the low- and high-Zn diets were enriched with 18% BT, SF, BT + SF (1:1 w/w), or olive oil (OL). Final live weights showed a Zn × fat source interaction. The low-Zn SF and OL groups consumed less food and weighed less than the low-Zn BT group. At the high-Zn level, final weights were comparable among groups. Serum and femur Zn concentrations did not significantly vary among groups fed diets with the same Zn level. Possibilities for the growth-retarding effect of the diets rich in unsaturated fatty acids (SF and OL) are discussed. Metabolic studies are needed to clarify the observed zinc × fat source interaction.