In five groups of 15 rats allogeneic kidney transplantations were performed. Four groups received pre- and postoperatively a semisynthetic diet, isocalorically but differing in quantity and quality of fatty acids: group I received a diet high in saturated fat; group II, a diet high in linoleic acid; group III, a diet containing fish oil; group IV, a diet high in monoenoic acid. Finally, the fifth group of rats was fed a standard commercial chow and served as a control for the procedure of technique and immunological regimen. All groups received the same immunosuppressive regimen of immunological enhancement induced by pretreatment with complete donor blood. Survival and several parameters of graft function were studied. The results showed that the technical mortality, i.e. animals dying in the first week after transplantation, was substantially higher in rats on the semisynthetic diets in comparison with the group of rats on the commercial diet. A statistically significant better graft function could be observed in the group of rats on the diet high in linoleic acid in the first period after kidney transplantation, compared to the other groups on semisynthetic diets. This difference disappeared in the course of the study when a number of animals was lost due to graft rejection. Furthermore, in the same diet group mortality due to rejection was significantly decreased as well.