16-days old rats were operated with either uni- or bilateral ligation of ductuli efferents and separation of testis and epididymis to the level of the inferior epididymal artery (non-union operation), induction of cryptorchidism or bilateral sham operation. The epididymides were weighed and the epididymides and deferent ducts were examined with light- and electron-microscopy at days 30, 37, 44 and 58. Bilateral non-union operated epididymides and cryptepididymides had a significantly lower weight increase than controls, but the histology and diameter of epididymal tubules were unchanged. This indicates a true growth retardation and reduced length of epididymal tubules of non-union operated and cryptepididymides. For bilateral operations a positive correlation was found between the weight of epididymis and plasma levels of total testosterone as reported earlier. Unilaterally operated epididymides had a weight development significantly below contralateral controls, despite normal plasma levels of testosterone. It is concluded that the reduced-weight of unilaterally operated epididymides is the result of diminished local androgen stimulation from the ipsilateral testis. Non-union of testis and epididymis may have pathogenetic significance in maldescent of testis by a retarded growth of the ductal system.