Tyrosine, a large neutral amino acid normally present in protein foods, is the precursor of the catecholamine neurotransmitters dopamine, norepinephrine, and epinephrine. Animal studies indicate that systemic administration of tyrosine in pharmacological quantities can reduce physiological and behavioral decrements induced by highly stressful conditions. The current study was designed to test the effects of tyrosine (100 mg/kg of body weight) on humans exposed to cardiovascular stress. Twenty participants were exposed to two Lower Body Negative Pressure (LBNP) sessions (-50 mm Hg for a maximum of 30 min) during each of two testing sessions of a repeated measure double-blind placebo-controlled study. The detected effects of tyrosine include an overall increase in pulse pressure (LBNP typically reduces pulse pressure) and an increase in auditory event related potential amplitude (P300-N300), an electro-physiological correlate of attention which may indicate enhanced cognitive activation.