The study used 56 testes collected from 28 Arab and native stallions (3-18 years) during a complete annual cycle. The consequences of seasonal and age-related changes in parenchymal weight and in numbers of spermatogonia, young and old primary spermatocytes, round spermatids, Sertoli cells and Leydig cells were evaluated. There were statistically significant seasonal and age effects on parenchymal weight, numbers of Sertoli cells, Leydig cells and spermatogenesis. Seasonal changes in the above testicular parameters were maximal in spring and winter, followed by summer, then reaching them minima in the autumn. The highest values for these criteria were reached by stallions of 6- <13 years, whereas the lowest values were observed later in life. Neither age nor season influenced the diameter of Sertoli cell nuclei, Leydig cell or Leydig cell nuclei. The mechanisms by which the numbers of both somatic testicular cells fluctuate with the yearly reproductive cycle of the stallion were discussed. Evidence from the present results revealed that numbers of Sertoli cells, germ cells and parenchymal weight were interrelated. Numbers of spermatogonia, Sertoli cells and Leydig cells accounted for 59% 58% and 48% of the variation in parenchymal weight, respectively. The relationships existed between the number of either of the two somatic testicular cells and spermatogenesis were also scrutinized. The present results emphasize the importance of age and seasonal changes in numbers of Sertoli cells and Leydig cells on regulation of stallion's spermatogenesis.