The testis of the donkey was used as a model to study age-related changes in the period between puberty and senium. From the age of 1.5 years to the middle of sexual maturity (5 to 6 years) a number of histophysiological features, all indicative of the spermatogenetic efficiency, increase continuously. Then without a longerlasting plateau of maximal performance these features undergo continuous retrogression. Thus, the adult testis is an organ in permanent change. During its progressive period (1.5 to 5 years) the average testicular and tubular volumes treble. The increase in tubular volume is due to an increase in tubular length (from 700 m to 1600 m per testis) and tubular diameter (from 205 pm to 250 pm). Parallel to this growth, the spermatogenetic efficiency of the seminiferous epithelium rises: the number of germ cells entering meiosis increases and the cell loss by apoptosis or exfoliation decreases. During the following regressive period of the testis (5-10 years) seminiferous epithelial height and tubular diameter are again gradually reduced to 70 pm and 205 pm, respectively. The absolute number of Sertoli cells per testis decreases continuously from puberty onwards. The tubular lamina propria thickens with advancing age and at the age of 10 years, displays long irregular projections into the seminiferous epithelium.