['The prospect of long-term residence in outer space is becoming a reality. Previously we found that testosterone secretion and spermatogenesis is reduced in model mice on which the shift of body fluids in space is simulated. The present study examines the effects of the microgravity (muG) environment on steroidogenesis of Leydig cells.', "I-10 cells were cultured for 24 h under a simulated muG environment. Progesterone production was evaluated as steroidogenesis. Western blots of the enzyme, 3betahydroxy dehydrogenase (HSD) as well as of the transcription factors, Ad4bp/SF-1 and DAX-1, the amount of adenosine3',5'-cyclic monophosphate (cAMP) and cell morphology were investigated. The muG environment was simulated using clinostat rotation (CR). Rotational (Cont-1) and stationary (Cont-2) controls always received a regular gravity effect with and without rotatory motion, respectively.", 'Progesterone levels in cells under CR were significantly increased compared with those of both controls. Increased progesterone production was not related to the expression of either of the transcription factors or the enzyme. Although the cytoplasm expanded, the amount of cAMP produced under CR did not significantly differ from that produced under both control environments.'] Does simulated conditions of microgravity increase progesterone production in I-10 cells of Leydig tumor cell line?