Experimental leptospirosis was reproduced in golden Syrian hamsters (Mesocricetus aureatus). Evaluation of three Leptospira serogroups (canicola, icterohaemorrhagiae and pomona) was carried out regarding their virulence, production of symptoms and lethal activity. Macroscopic and microscopic studies of tissue lesions were also made. Animals were inoculated with 3.75 x 10(6) to 6 x 10(7) bacteria through the intraperitoneal (IP) route. The clinical development of the disease in Syrian hamsters was characterized by the presence of general symptoms between the third and fifth day after inoculation. With highly virulent strains, the distinctive clinical picture occurred and death followed before the seventh day. The strain of the pomona serogroup proved to be more virulent than those of the other serogroups and produced the most florid picture of the disease. Pathogenic aspects of this process were also studied.