Acute oral administration of the catecholamine precursor tyrosine is associated with increased exercise capacity in the heat. It is unclear whether exercise performance in the heat is improved following acute tyrosine administration. To explore this possibility, seven male endurance-trained volunteers [median age, 20 (range 19 – 45) years; mean stature, 1.82 ± 0.05 (SD) m; body mass, 77.9 ± 11.7 kg; VO2peak, 4.6 ± 0.6 l min-1], unacclimated to exercise in the heat, performed two tests in a randomised crossover fashion, separated by at least 7 days. Following familiarisation, subjects were assigned in a double-blind fashion, 500 ml of sugar-free lemon and lime flavoured water with 150 mg kg body mass-1 tyrosine (TYR), or the same volume of sugar-free flavoured water with an isocaloric quantity of hydrolysed whey powder (Whey). After 1 h subjects cycled at a constant exercise intensity (57 ± 4% VO2peak) for 60 min then performed a simulated cycling time trial (TT), requiring the completion of an individualised set work amount (393.1 ± 39.8 kJ) as quickly as possible, in 30°C and 60% relative humidity. Normally distributed data were analysed using repeated measures (time × trial) analysis of variance (ANOVA), and post hoc paired Student’s t-tests with the Bonferroni correction. Data not normally distributed were analysed using Friedman’s test and post hoc Wilcoxon matched-pairs tests. The plasma ratio of tyrosine + phenylalanine:Σ(free tryptophan, leucine, isoleucine, valine, methionine, threonine, lysine) exhibited an interaction (P < 0.001; ANOVA, n = 7), had increased over 2.5-fold at pre-exercise compared to rest with TYR (P < 0.001; n = 7), and remained elevated from rest throughout 60 min of submaximal exercise and at end of TT (P < 0.001 in all cases; n = 7), whereas it had declined in Whey at pre-exercise (P = 0.004; ANOVA, n = 7). The plasma ratio of free tryptophan:Σ(phenylalanine, tyrosine, leucine, isoleucine, valine, methionine, threonine, lysine) had declined at pre-exercise in TYR (P < 0.01; ANOVA, n = 7) but was unchanged in Whey at any timepoint (P > 0.05; ANOVA, n = 7). Mean power output throughout TT was 198 ± 41 W in TYR and 191 ± 46 W in Whey (P = 0.869; ANOVA, n = 7), therefore time to complete TT (P = 0.4167; paired Student’s t-test, n = 7) was similar in both trials (34.8 ± 6.5 min and 35.2 ± 8.3 min in TYR and Whey respectively). There was no difference between trials in RPE (P > 0.05; Friedman’s test, n = 7), thermal sensation (P > 0.05; Friedman’s test, n = 7), core temperature (P = 0.860; ANOVA, n = 7), skin temperature (P = 0.683; ANOVA, n = 7), or heart rate (P = 0.314; ANOVA, n = 7). These data indicate that acute tyrosine administration does not improve simulated TT performance in the heat. The lack of effect compared to a capacity trial to exhaustion may be related to the inherent self-paced nature of TT.