The lactate dehydrogenase (LDH) isoenzyme pattern of prostate and of testis of 27 wild animals showed that in several species more than 5 isoenzymes are detected, with electrophoretic mobilities different to those found in humans. The LDH-X band, found in testis from mature humans is also observed in the testis of wild animals. However, in several species more than one LDH-X is found. The results obtained demonstrate their usefulness in phylogeny. The prostate gland and testis of the chimpanzee show the greatest resemblance to man. Histological examination of the prostate glands seems to correlate with the phylogenetical classification of the mammals studied.