PURPOSE: Evaluate the effects of finasteride on the serum PSA and on the prostate of hamster-Mesocricetus auratus(hMa). METHODS: Twenty hMa male adults were split in groups control and experimental (n=10). Animals of the experimental group received 7.14ng/mL of finasteride, subcutaneously (SC) on the back three times per week, during 90 days. The finasteride dose was equivalent to 5.0mg administered to a 70kg man. At the end of the experiment the mean age for the animals in the control group was 15.2 + or - 1.13 months and for the experimental group was 17.7 + or - 0.67 months. There was a statistically significant difference between mean ages of both groups (t value=5.98; p=0.001). The animals of the control group weighted 129.0 + or - 18.8g and the experimental group weighted 145.0 + or - 15.5g, t=1.88 e p=0.0514. The serum PSA was assessed through ELISA method. Prostates of those animals were collected and processed to histology and morphometry: the diameter of the acinous glands and the acinous epithelium, apoptosis, AgNORs and cellularity were assessed in both groups. RESULTS: Serum PSA decreased in the experimental group, 0.003ng/mL versus 0.763ng/mL, H= 7.982 e p= 0.0047. Decrease in the acinous area occurred in animals that received finasteride, 238.000 + or - 24.600 microm(2) versus 398.600 + or - 55.320 microm(2); t= 2.653; p= 0.0122. A remarkable decrease in the area of the acinous epithelium occurred in the animals that received finasteride, 111.900 + or -12.820 microm(2) versus 160.400 + or - 18.430 microm(2) t= 2.162; p= 0.0361. AgNORs were less expressed in finasteride treated animals, 2.846 + or - 0.877 versus 3.68 + or - 1.07 argyrophilic clusters for microm(2), p= < 0.0001. Apoptosis was more intense in the experimental group, 53.62 + or - 1.389 than in controls, 14.76 + or - 2.137, p= 0.0408. However, there was no statistical difference in the cellularity between both groups, 74.75 + or - 5.5 cells, in controls versus 65.07 + or - 13.24, in treated animals, p=0.5105. CONCLUSIONS: Use of finasteride decreased serum PSA and several histological parameters of the Hamster's prostate, such as lumen area, acinous and epithelium area, and AgNORs expression. Finasteride increased apoptosis in the prostate acinous cells although no significant difference in the cellularity could be found between the two groups of animals under study.