The present study assesses interobserver variability and day-to-day reproducibility of ventilatory threshold T(ven) and lactate threshold T(lac) in healthy young men. The data were obtained before and after acute beta-adrenergic blockade with metoprolol (100 mg/d p.o.). Fifteen healthy young men underwent progressively increased treadmill ergometry tests on two separate days (day 1, 2). Ten out of fifteen subjects participated in a third exercise test under an acute beta-adrenergic blockade (day 3). Interobserver variability of oxygen uptake at T(ven) (8%; +/- 0.136 l/min) and T(lac) (5%; +/- 0.984 l/min) was small and day-to-day reproducibility of T(ven) (7%; +/- 0.131 l/min) and T(lac) (7%; +/- 0.133 l/min) high. Under acute beta-adrenergic blockade T(ven) occurred at a significantly lower work load (360 +/- 117 s) as compared with T(ven) of days 1 and 2 (477 +/- 153 s; p < 0.05), and T(lac) of day 3 after beta-adrenergic blockade (456 +/- 76 s; p < 0.05). Therefore, a significantly lower oxygen uptake (1.409 +/- 0.29 l/min) could be observed at T(ven) of day 3 as compared with T(ven) of days 1 and 2 (1.852 +/- 0.30; p < 0.001), and T(lac) of days 1 and 2 (1.724 +/- 0.22; p < 0.001). There was a significant linear correlation between oxygen uptake at T(ven) and T(lac) before beta-adrenergic blockade (r = 0.86; p < 0.001). This correlation, however, was lost following an acute beta-adrenergic blockade (r = 0.56; n.s.).(ABSTRACT TRUNCATED AT 250 WORDS)