Changes in the hematological picture in newts have been investigated utilizing benzene exposure. The first deviation occurs in circulating white cells that become more numerous. Subsequently there is an increase of immature cells on the cortical zone of the liver. About two weeks after the first treatments a decrease of circulating leukocytes begins and the immature white cells continue to increase both in circulation and in the cortical zone of the liver. The red cells present a decrease after one month of treatment. This is preceeded by a decrease in hemoglobin percentage and an increase in the size of red cells and in the number of immature red cells. The data concerning animals treated with an hemolytic agent and with benzene demonstrates an early action of benzene also on red cells. The results suggest a maturation in capacity caused by benzene in both white and in red cells. The intensity of alteration seems proportional to the frequency of treatments. The onset of a particular modification, however, is related to the time interval following the first treatment.