In order to obtain information on the vitamin B2 status in athletes in training, the following aspects were investigated: in one group of athletes, riboflavinuria at rest, during the autumn and winter months (mean values 23.8 and 9.45 mug/h, respectively); in the second group (n = 18), the activity of the nicotinamide-adenine-dinucleotide-phosphate-dependent red blood cell glutathione reductase (EC 1.6.4.2) and its activation by added flavin adenine dinucleotide. In 8 athletes, activation coefficients higher than 1.25 were found, indicating a biochemical deficiency of riboflavin. Erythrocyte transketolase (EC 2.2.1.1) was activated by more than 25% with added thiamine pyrophosphate in 12 cases. In muscle samples of 10 healthy patients (fresh fractures or removing of metal plates from old fractures), glutathione reductase (mean activity 0.846 +/- 0.458 U/g wet weight) was activated by more than 25% in 5 cases. Administration of 10 mg of riboflavin per os resulted in moderate lowering of neuromuscular irritability (musculi vastus medialis and lateralis quadricipitis) particularly for 0.1-msec rectangular stimuli.