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    The Bioastronautics Roadmap risks include “unacceptable levels of increased hereditary, fertility, or sterility risk caused by occupational radiation exposure or the combined effects of radiation and other space flight factors.” One potential countermeasure to the effects of microgravity (µG) is artificial gravity (AG) using centrifugal forces. We examined the effect of AG (2G) on testis development and fertility in male rats. Pregnant dams were exposed to continuous 2G from gestational day 9 (G9), parallel pregnant dam controls were Rotational controls (RC) and stationary controls (SC). At birth, liters were adjusted to 8 male and 2 female pups per litter and maintained for an additional 21 days of post-natal development in continuous AG, RC or SC conditions, respectively. Testes were harvested at 21, 45, and 60 days of age, and mating trials were run at 45 and 60 days of age. Testis and whole body weight were significantly lower in HG vs RC and SC animals at all times. Histology of the testes showed a delayed onset of spermatogenesis in the 45 day old HG rats vs RC and SC. Gene array analysis of 21 day old testis RNA revealed significant upregulation of Hsp90-beta (critical for late stage spermatogenesis), and ATRX (a gene critical for developmental conversion of the female to male gonad), and significantly lower transcription of Hsp70t (a testis-specific gene critical for late stage spermatogenesis) and Hsp70BP and Hsp40 (both complexed with Hsp’s for function during meiosis).  Spermatogonial (SG) apoptosis was also significantly higher in the 21 day old HG testis vs RC and SC, which corresponds to a delay in the normal apoptotic reduction in SG populations that occurs ~5 days earlier during normal testis development. All HG, SC and RC animals failed to produce litters and 45 days, however all animals in all groups produced normal litters by 60 days of age. These data suggest that AG during prenatal and early post-natal development causes a delay in male puberty that recovers even though testis and body weight are compromised. (Supported by NASA and NIH).