In the past the hypothesis was advanced that plasma tryptophan fractions mediate behavioural and neuroendocrine effects of exercise. To assess changes in free and total plasma tryptophan levels during and after exercise over a time period, and possible functional implications, 6 male volunteers were subjected to an endurance test of 1 hour duration on a bicycle ergometer at a work load below the 4 mmol/l lactate threshold. Total and free plasma tryptophan were determined by high-performance liquid chromatography with electrochemical detection, prolactin by radioimmunoassay, glycerol and nonesterified fatty acids (NEFA) by enzymatic methods. No significant changes in total tryptophan were found. Free plasma tryptophan increased significantly (23%, p less than 0.01) from 40 to 60 minutes of exercise. Highest levels were observed after 10 minutes of recovery (50% against preexercise). Prolactin levels correlated with free plasma tryptophan throughout the test (r = 0.77, p less than 0.001, all measured values) and during recovery (10 minutes: r = 0.88, p less than 0.05; 20 minutes: r = 0.86, p less than 0.05). These findings may further support the hypothesis that changes in peripheral amino acid concentrations may influence physiological reaction of exercise mediated by brain systems.