A combination of tyrosine, capsaicin, catechins, and caffeine has been shown to possess a thermogenic effect in humans. The present objective was to investigate whether the thermogenic response to the bioactive combination (BC) could be diminished or abolished by propranolol. Twenty-two men (age, 29.0 +/- 7.1 years; body mass index, 26.0 +/- 3.6 kg/m(2); mean +/- SD) participated in a 4-way, randomized, double-blind, placebo-controlled crossover study. The effect of the following was tested: (1) placebo, (2) BC, (3) BC + 5 mg propranolol, and (4) BC + 10 mg propranolol. Resting metabolic rate, respiratory quotient, and the thermogenic response were measured for 5 hours postintake. Systolic blood pressure (SBP), diastolic blood pressure (DBP), heart rate, and appetite ratings were assessed every half hour. The BC increased resting metabolic rate by 5% (73 [36; 110] kJ/5 h, mean [95% confidence interval], P < .0001) compared with placebo. Both propranolol doses blunted the thermogenic response by 50% compared with placebo (P < .01). The BC increased SBP by 3% (4 +/- 1 mm Hg, P = .003) compared with placebo. The effect of BC on SBP was reduced by 25% by propranolol (P = .07). The BC (with or without propranolol) increased DBP by 6% (4 +/- 1 mm Hg, P </= .0002). Propranolol decreased heart rate by 5% (3 +/- 1 beats per minute, P < .0001) compared with placebo and BC. No effects were observed on appetite ratings. In conclusion, the study confirms the thermogenic properties of BC. The 50% reduction of the thermogenic response by propranolol indicates that beta-adrenergic pathways are partly responsible for the thermogenic response.