The fatigue and cognitive performance deficits associated with sleep loss and stress, like that experienced during sustained flight operations and nighttime flying, have motivated the search for effective nonpharmacological countermeasures. The behavioral effects of the potential countermeasure tyrosine, an amino-acid precursor to dopamine and norepinephrine, were examined during an episode of continuous nighttime work involving one night's sleep loss. Volunteers performed nine iterations of a battery of cognitive and subjective tasks for approximately 13 h, beginning at 1930 and ending at 0820 the following morning. Subjects remained awake throughout the day on which the experiment began and were awake for approximately 24 h by the end of testing. Six hours after the start of the experiment, one-half of the subjects received 150 mg/kg tyrosine in a split dose while the other half received a cornstarch placebo in a double-blind procedure. The tracking-task performance of tyrosine subjects declined less during the night than that of placebo subjects. Tyrosine administration was also associated with nonsignificant trends toward reducing (a) lapses on a high-event-rate vigilance task, (b) subjective sleepiness, and (c) the intensities of several fatigue-related symptoms.