Aging is known to disrupt the "biological clock" that governs physiological variables at rest. This study sought to determine whether aged men demonstrated biorhythmic variation in muscle performance during resistance exercise and physiological responses to that stimulus. Ten aged (75.6 ± 1.6 yrs; mean ± SE) men completed an isokinetic testing protocol of knee extensors and flexors at 0800, 1200, 1600, and 2000 h. While time of day variation in peak torque was detectable, significant (P 0.05) oscillation was established only in the knee flexors at 3.14 rad/s. Heart rate, blood pressure, and rectal temperature displayed no significant variation, but trends (P < 0.10) in oscillation of post-exercise blood pressure and rectal temperature were noted. Temporal patterns in biorhythmic variation of muscle performance, as well as thermal and cardiovascular measures, emulated those observed in a previous study involving young men where the magnitude of variation was sufficient to achieve statistical significance. Similar to our earlier findings in young men, however, pre- and post-exercise testosterone and cortisol concentrations demonstrated significant variation among aged men. These data confirm the blunting of biorhythmic variation in muscle performance and physiological variables, except for circulating hormones, in aged men.