We studied the action of whole rattlesnake venom on red blood cells and leucocytes of adult male and female rats. Animals were surgically cannulated for blood collection directly from the inferior caval vein and injected intramuscularly in the thigh with a mixture of venoms from a large number of rattlesnakes. The signs shown by the animals were paralysis of the hind part of the body, lack of motor coordination, and respiratory difficulties, with death occurring in some cases. Necroscopy showed petechial hemorrhage in the intestine and jejunum and darkening of the viscera, which was found to be due to engorged blood vessels upon histopathological examination. Blood examination showed a change in color to dark brown due to the transformation of hemoglobin to methemoglobin. Venom fractions were found to have a low hemolytic power because of their low concentration in the venom samples. Blood sedimentation rate showed a clear variation, especially 60 minutes after venom injection. Both phenomena may be linked to the lytic power of the venoms. An interesting phenomenon was that the animals showed initial leucopenia, which was followed by persistent leucocytosis. Lymphocytopenia and increased neutrophil numbers were also observed. The present results led us to conclude that rattlesnake venom has a relative hemolytic power which increases with venom concentration and with the concentration of the fractions in whole venom.