Abstract. The present study demonstrates the postnatal developmental changes in immunohistochemical localization of α-smooth muscle actin (SMA) and vimentin in the bovine testis. In the peritubular myoid cells of seminiferous tubules and the sub-epithelial and stromal cells of straight tubules and the rete testis, α-SMA starts appearing at around 4 months of age. Peritubular α-SMA attains the continuous mature pattern at around 5 months of age whereas sub-epithelial and stromal α-SMA increases with advancing age. Vimentin is localized in the perinuclear zone of Sertoli cells, peritubular and vascular wall cells, a few interstitial cells, and in the basal part of the epithelia of straight and rete tubules. Developmental changes are only evident in the Sertoli cell vimentin, which is basal and weak at birth and increases moderately until 4 months of age. From around 5 to 8 months of age when the Sertoli cells are under morphological transformation, vimentin intensity is considerably increased and the characteristic vimentin extensions connect the Sertoli nuclei to the basal membrane. These extensions get shorter at around 9 month of age as the Sertoli nuclei are positioned basally. The mature Sertoli cell perinuclear vimentin is strong and stable without infranuclear extension. In conclusion, the age of appearance of α-SMA coincides with the onset of postnatal division of spermatogonia, and vimentin may play a key role in stabilizing Sertoli cell nuclei during their transformation in bovine. positioning of intracellular organelles, forming of cytoplasmic extensions, and anchoring of organelles to the plasma membrane [1]. As a specific marker of smooth muscle differentiation [2], seminiferous peritubular α-smooth muscle actin (SMA) has been detected in the first few postnatal days in the rat testis [3,4] and in the early postnatal ovine testis [5]. Increasing patterns of α-SMA in the ovine testis [5] and changes in the actin filament arrangement in the rat testis [6] during p o s t n a t a l d e v e l o p m e n t h a v e s u g g e s t e d a relationship for SMA with testicular development and functions. However, no postnatal localization patterns or possible functional relationships for SMA in the bovine testis have been reported so far.In the testis, Sertoli cells support germ cells and also constitute the seminiferous tubules. Sertoli cells posses a highly organized cytoskeleton, and vimentin, the most common intermediate filament [7,8], is located around the nucleus and provides it with structural support [5,[9][10][11]. During pubertal processes, mammalian Sertoli cells undergo transformation [12,13] that is likely related to cytoskeletal changes [11,14]. Increases in Sertoli cell vimentin in the prepubertal ovine testis [5] and changes of its distribution area during postnatal development in rats [11] have indicated vimentin activity during postnatal development. Although perinuclear localization of vimentin has been demonstrated in the pre-Sertoli and adult Sertoli cells of the bovine testis [10,15], its dist...