BACKGROUND Spermatogenesis and epididymal function depend on testicular steroids with estrogens being important regulatory factors. However, testicular estrogen secretion shows distinct species specificities, with the boar being characterized by the production of high amounts of estrone [E1] and estronesulphate [E1S]. As the boar testis also expresses estrogen sulphatase [StS] and sulphotransferase [EST] the present paper is based on the hypothesis that local availability of biologically active estrogens results from an interplay between estrogen synthesis and local activities of StS and EST. METHODS Blood was collected during castration of 37 boars, aged between 98 (peripubertal) to 2 793 (old sexually mature) days, from the testicular vein and artery and peripheral circulation; E1, E1S, testosterone [T] and progesterone [P] were determined by established RIA-procedures. Similarly seminal plasma from 21 sexually mature boars was assessed. StS- and EST-activity were determined in testicular- and epididymal homogenates of 3 sexually mature boars (200 d) using (3)H-E1S resp. (3)H-E1 as substrate. Immunohistochemistry [IHC] was applied to locate EST- expressing cells in testes in relation to age and in the epididymis of 3 mature boars. RESULTS Hormone data showed a high variability. A significant age dependent increase was only observed for E1 and E1S in the peripheral circulation with absolute values being highest for E1S (5-60 nmol/l), followed by T (2.6-14 nmol/l), P (0.5-1.48 nmol/l) and E1 (0.24-0.84 nmol/l). Testicular vein concentrations always exceeded those in the testicular artery with the differences being significant for E1 and P, group 1. Concentrations in the testicular artery and peripheral vein plasma were not different but higher (p<0.0001) than those in seminal plasma with the exception of E1. StS activity was higher (p<0.001) in the testis than the epididymis. EST activity was high in epididymal homogenates and at the level of detection in testis homogenates. IHC located EST in virtually all epididymal epithelial cells. In the testis the number of positive staining Leydig cells decreased (p<0.05) from 72% in the premature to 57% in the mature boars. CONCLUSIONS The provision of biologically active estrogens to the testicular and epididymal compartment is controlled by a complex regulatory system, with the sulphatase pathway being an important component. P is a secretory product of the testis, E1 and E1S are not actively enriched in seminal plasma.