The study was conducted on female and male Swiss mice with body mass of 20-30 g. The experimental animals were administered deltamethrin in concentrations of 5 mg/kg b.m. (1/10 LD₅₀) and 25 mg/kg b.m. (1/2 LD₅₀), or fenvalerate in the doses of 10 mg/kg b.m. (1/10 LD50) and 50 mg/kg b.m. (1/2 LD₅₀). Pyrethroids were administered intragastrically once a day for 28 days. Parallel studies were conducted in two control groups. The following organs were taken for histologic examinations: liver, kidney, lung, heart and spleen. Blood was taken from the heart for hematologic tests. The total number of erythrocytes and leukocytes, the percentage of neutrophils and lymphocytes, as well as the level of hemoglobin and hematocrit were determined. Deltamethrin and fenvalerate caused degenerative changes in the liver and kidneys in Swiss mice. Changes were more intense in male mice which were administered deltamethrin, and in female mice which received fenvalerate. Irrespective of the dose, the pyrethroids examined stimulated erythropoiesis and synthesis of hemoglobin in male Swiss mice, while in female mice the administration of deltamethrin in the dose of 1/10 LD₅₀ resulted in the suppression of erythropoiesis and hemoglobin synthesis. Both in male and female mice, deltamethrin and fenvalerate-irrespective of the dose-caused a general increase in the number of leukocytes.