Cultured STe cells (2n karyotype) from swine testis were submitted to simulated microgravity using a 3D Random Positioning Machine for 5 min., 15 min., 30 min., 1 h and 23 h. Sample processing included: histological characterization of cell types, immunohistochemical identification of (i) microtubules (a-tubulin), (ii) alkaline phosphates, (iii) 3 beta-hydroxy-steroid-dehydrogenase (3?-HSDH), and histochemical lipid analyses. After 5 min. simulated microgravity a slight microtubule disorganisation occurred, which increased dramatically with increasing microgravity duration. After 23 h microtubule arrays were completely disrupted. 3 beta-HSDH immunostaining was detectable only in one cell type: under control conditions and 5 min. into microgravity immunoreactivity was strong, but completely disappeared thereafter. Immunostaining intensity for alkaline phosphates, a good marker for myoid cells, decreased after 15 min. in microgravity.