Lower testicular tetosterone:17β-estradiol (T:E2) ratio was found in teratospermic domestic cats (<40% morphologically normal sperm). The aim of this study was to assess the reliability of the tritiated waterrelease assay (TWRA) to measure aromatase activity in domestic cat testes. Testicular T and E2 concentrations, measured by enzyme immunoassay, and sperm morphology were evaluated to verify the relationship between them. Aromatase activity was measured in microsomal fraction and in homogenates of cat testes. Rat ovaries and piglet testes were used for assay validation. Aromatase activity was not detected in cat testes microsomal fraction (n = 8), not even when the protein amount added to the assay was increased from 50 to 200 µg. In homogenates, however, it was detected (3.5 ± 0.5 pmol.g-1 .h-1 ; n = 7), although in such low levels that no activity inhibition was detected when homogenates were incubated with increasing fadrazole concentrations. Although none of the cats in this study were classified as teratospermic, some sperm defects were correlated with testicular T:E2 ratio (abnormal acrosome, r = -0.76) and with E2 concentration (proximal cytoplasmic droplet, r = 0.77). However, we did not find any correlation between aromatase activity and hormonal or sperm morphology data. To our knowledge this is the first demonstration of testicular aromatase activity in domestic cats. Despite that, due to the low aromatase activity measured and the lack of correlation with other reproductive data, we could not infer that TWRA is a reliable method to detect differences in testicular aromatase activity in normospermic cats. Perhaps this method could be used in teratospermic individuals that probably have an increased aromatase activity. As an alternative, we suggest that more sensitive techniques should be used to compare aromatase activity between normospermic and teratospermic cats. This would allow a better understanding of the relationship between the level of aromatase activity, the testicular hormonal concentrations and the sperm abnormalities in domestic cats.