Endocrine and neuromuscular effects of prolonged strength training were investigated in 21 strength-trained male subjects during the course of a 24-week progressive strength training and during a subsequent detraining period of 12 weeks. Maximal isometric leg extensor force increased by 19% (P less than 0.001) during the first 20 weeks, followed by a plateau during the 4 latest weeks of training. During the course of the training period, no systematic change was found in serum testosterone concentrations, but there was a decreasing tendency in the concentrations of free testosterone (NS), 17-OH-progesterone (NS), androstenedione (P less than 0.05), dehydroepiandrosterone (P less than 0.05), cortisol (P less than 0.01), transcortin (CBG) (P less than 0.05), and in the cortisol/CBG ratio (P less than 0.05). The last 4 weeks of training were characterized by significant correlations between the individual changes in maximal isometric force and the changes in serum free testosterone concentrations (r = 0.60, P less than 0.01). The changes in the ratios of free testosterone to cortisol (r = 0.73, P less than 0.001), total testosterone to cortisol (r = 0.83, P less than 0.001), and 17-OH-progresterone to cortisol (r = 0.62, P less than 0.01) also correlated with the changes in maximal force. The findings suggest that the turnover of endogenous androgens may increase during progressively intensified training without a change in serum total testosterone concentration. Prolonged intensive strength training may also lead to changes in the concentrations of serum cortisol and transcortin. During the most stressful phases of training, the changes in serum androgen/cortisol ratios seem to be highly individual and may correlate with changes in muscular strength.