Spaceflight and prolonged bed rest (BR) alter plasma hormone levels inconsistently. This may be due, in part, to prescription of heavy exercise as a countermeasure for ameliorating the adverse effects of disuse. The initial project was to assess exercise programs to maintain aerobic performance and leg strength during bed rest. The present study evaluates the effect of BR and the performance of the prescribed exercise countermeasures on plasma steroid levels. In a 30-day BR study of male subjects, the efficacy of isotonic (ITE, n=7) or isokinetic (IKE, n=7) exercise training was evaluated in contrast to no exercise (NOE, n=5). These exercise countermeasures protected aerobic performance and leg strength successfully. Bed rest alone (NOE) did not change steroidogenesis as assessed by the plasma concentrations of cortisol, progesterone, aldosterone, and free (FT) and total (TT) testosterone. In the exercise groups, both FT and TT were decreased (P<0.05); FT during IKE from 24 ± 1.7 to 18 ± 2.0 pg^.ml^-1, and during ITE from 21 ± 1.5 to 18 ± 1 pg^.ml^-1; and TT during IKE from 748 ± 68 to 534 ± 46 ng^.dl¹, and during ITE from 565 ± 36 to 496 ± 38 ng^.dl^-1. The effect of intensive exercise countermeasures on plasma testosterone was not associated with indices of over-training. The reduction in plasma testosterone associated with both the IKE and ITE countermeasures during BR, support our hypothesis that intensive exercise countermeasures may, in part, contribute to changes in plasma steroid concentrations during spaceflight.