Effects of 6 months of heavy resistance training combined with explosive exercises on both basal concentrations and acute responses of total and free testosterone, growth hormone (GH), dehydroepiandrosterone (DHEA), dehydroepiandrosterone sulfate (DHEAS), cortisol and sex hormone-binding globulin (SHBG), as well as voluntary neural activation and maximal strength of knee extensors were examined in 10 middle-aged men (M40; 42 +/- 2 years), 11 middle-aged women (W40; 39 +/- 3 years), 11 elderly men (M70; 72 +/- 3 years), and in 10 elderly women (W70; 67 +/- 3 years). The maximal integrated electromyographic (iEMG) and 1 repetition maximum (RM) knee-extension values remained unaltered in all groups during a 1-month control period with no strength training. During the 6-month training the 1RM values increased in M40 by 27 +/- 9% (p < .001), in M70 by 16 +/- 6% (p < .001), in W40 by 28 +/- 11% (p < .001), and in W70 by 24 +/- 10% (p < .001). The iEMGs of the vastus lateralis and medialis muscles increased(p < .05-.001) in M40, M70, W40, and W70. No systematic changes occurred during the experimental period in the mean concentrations of serum total and free testosterone, DHEA, DHEAS, GH, cortisol, or SHBG. However, the mean levels of individual serum free testosterone in W70 and serum testosterone in the total group of women correlated with the individual changes recorded in strength during the training (r = .55,p <.05; and r = .43,p <.05). The single exercise session both before and after the training resulted in significant responses in serum total and free testosterone concentrations in both male groups (p <.05-.01), but not in the female groups, as well as in serum GH levels in all groups (p <.05-.01) except W70 (ns). In summary, the present strength training led to great increases in maximal strength not only in middle-aged but also in elderly men and women. The strength gains were accompanied by large increases in the maximal voluntary activation of the trained muscles. None of the groups showed systematic changes in the mean serum concentrations of hormones examined. However, a low level of testosterone, especially in older women, may be a limiting factor in strength development and testosterone could mediate interactions with the nervous system contributing to strength development. The physiological significance of the lack of acute responsiveness of serum GH to heavy resistance exercise in older women for their trainability during prolonged strength training requires further examination.