The levels of testosterone (T) and 17-beta-estradiol (E2) in seminal plasma were determined by the direct RIA method with tritium-labelled testosterone and 17-beta-estradiol. Testosterone was determined in 47 ejaculates and E2 in 132 ejaculates of seven bulls whose age ranged from 10 to 26 months, and in 60 ejaculates of two breeding boars old 20 months. The seminal plasma of bulls was found to contain 2.09 +/- 1.67 nmol/l testosterone and 2.75 +/- 1.94 nmol/l E2. Without respect to the age of the sires, the marginal values of the studied steroids showed a comparatively high fluctuation so that the average values are not very different between individual animals. No relation was found between the level of steroids and the concentration of fructose; this applies to the fertile bulls as well as to the bull suffering from epididymitis with the formation of epididymal cysts. Breeding boars had 0.338 nmol/ E2 (+/- 0.3) and 6.40 +/- 4.01 nmol/l testosterone in their seminal plasma. When the obtained values are recalculated to an average ejaculate volume, 5 ml of the seminal plasma of bulls will contain about 0.013 nmol E2 and 0.010 nmol T and 300 ml of the seminal plasma of boars will contain about 0.101 nmol E2 and 1.920 nmol T.