The Mustelidae family is one of the most diverse and numerous ones in the Carnivora order and yet, our knowledge about their reproductive biology is scarce. This study was designed to describe and compare the basic characteristics of testicular morphometry of 70 male American mink, taking the seasonal effect into consideration. Additionally, it was tested whether the size of the testes is related to sperm concentration. Mink testes were collected during the reproductive season and out of it. The testes were measured with a calliper and weighted on a precision scale. Spermatozoa were obtained by epididymal slicing and all semen samples were analysed using a CASA-system. A strong correlation between seasons of the year, testicular morphometry and sperm production was found. An additional index was created to normalize the collected data and compare it, showing statistically significant difference between the males in and out of the reproductive season. The average length and width of the testicle from males in breeding season was, respectively, ~1.9 and ~.1.4 cm, compared to ~1.3 and ~0.9 cm in non-breeding males. Testicles collected out of the breeding season had less biomass than the ones collected during the breeding season (~0.7g out of the season and ~3.8g in the season). The study provides new data concerning testicular morphometry in American mink. Their belonging into the seasonal breeders has been further confirmed, indicated by the changes in testes morphometry, as well as cease of sperm production in late autumn/winter.