Both testosterone (T) and cortisol (C) exhibit circadian rhythmicity being highest in the morning and lowest in the evening. T is a potent stimulator of protein synthesis and may possess anti-catabolic properties within skeletal muscle, and C affects protein turnover, thereby altering the balance between hormone-mediated anabolic and catabolic activity. Physiological reactions of these hormones and training adaptations may influence the post-exercise recovery phase by modulating anabolic and catabolic processes, therefore affecting metabolic equilibrium, and may lead to intensification of catabolic processes. We investigated the effect of the circadian system on the T and C response of weight-trained men to heavy resistance exercise. Thirteen young (21.8 +/- 2.2 yr) weight-trained men (12 months training experience) performed an eight-station heavy-resistance exercise protocol on two separate occasions (AM: 06:00 h and PM: 18:00 h), completing 3 sets of 8-10 repetitions at 75% of each subject's one-repetition maximum (1-RM). Blood samples were obtained prior to, during, and following the exercise bout, and serum total T and C concentrations were determined by competitive immunoassay technique. Performing the single bout of heavy-resistance exercise in the PM as compared to the AM positively altered the C and T/C ratio hormonal response. Pre-exercise C concentrations were significantly lower (p < 0.05) in the PM session, which resulted in a lower peak value, and the accompanying increased T/C ratio suggested a reduced catabolic environment. These data demonstrate that the exercise-induced hormonal profile can be influenced by the circadian time structure toward a profile more favorable for anabolism, therefore optimizing skeletal muscle hypertrophic adaptations associated with resistance exercise.