Male and female rats of Wistar strain were fed from the 6th week of life on during 10 and 30 days the diet containing 4.5% or 26% protein without or with addition of chlorfenvinphos (Chl) in concentrations of 5, 100 and 1000 ppm. The following was stated: In the case of optimal-protein diet (26% protein): the inhibitory effect of Chl on the body weight gain at its highest concentration; the increased relative weight of liver, ovaries and adrenals dependent on the amount of the insecticide in the diet. In the case of low-protein diet (4.5% protein): the reduced rate of the body weight gain already at 5 ppm, pronounced at 100 ppm and strongly marked at 1000 ppm of Chl; increase of the relative weight of testicles, adrenals and decrease of the relative weight of the spleen connected with the increasing concentrations of Chl. In rats with the most decreased rate of the body weight gain (in case of Chl) an increase of the relative weight of the brain. Congestion of the heart, spleen and kidneys after 30 days on diets containing 100 and 1000 ppm of Chl, independent of the amount of protein in the diet. Eosinophilic infiltrations in the lungs of rats obtaining the 100 ppm-contaminated low-protein diet during 30 days. Focal degenerative changes in the cells of the exocrine pancreas and proliferation of lymphatic nodules in the spleen mainly in rats receiving the 1000 ppm-contaminated optimal-protein diet for 30 days. After withdrawal of Chl from the diets changes caused by this insecticide showed reversal trends.