The purpose of this investigation was to study the relationship between both blood ammonia thresholds (AmT) and lactate thresholds (LT) during dynamic exercise in cardiac transplant patients (CTPs). Eleven male patients who had undergone orthotopic cardiac transplantation (age: 54 +/- 11 years, mean +/- SD; height: 165.1 +/- 6.6 cm; body mass: 78.3 +/- 16.1 kg) participated in this study. Each of them performed a bicycle ergometer test (ramp protocol) until volitional fatigue. During each test, gas exchange parameters and ECG responses were determined continuously. In addition, blood lactate and ammonia concentrations were measured every 2 min for determination of both LT and AmT, respectively. Peak values of oxygen uptake (Vo2), respiratory exchange ratio, ventilation, and heart rate averaged 15.9 +/- 3.03 mL.Kg-1.min-1, 1.02 +/- 0.06, 46.69 +/- 5.69 L.min-1, and 124 +/- 16 beats per minute, respectively. However, blood concentrations of lactate and ammonia at peak exercise were 3.7 +/- 0.4 mmol.L-1 and 85.6 +/- 31.7 micrograms.dL-1, respectively. LT and AmT were detected in 8 (72.7% of total) and 9 (81.8% of total) of 11 subjects, respectively. No significant differences were found between mean values of LT and AmT, when both were expressed either as Vo2 (10.01 +/- 1.19 vs 10.5 +/- 2.38 mL.kg-1.min-1, respectively) or as percent Vo2 peak (64.62 +/- 11.362 vs 66.48 +/- 9.19%, respectively). In addition, LT and AmT were significantly correlated (p < 0.05) when both were expressed either as Vo2 (mL.kg-1.min-1) or as percent Vo2 peak (r = 0.70 and r = 0.68, respectively). Our findings suggest that in CTPs, both LT and AmT occur at similar workloads, probably as a result of skeletal muscle alterations associated with chronic deconditioning and immunosuppressive therapy.