A series of experiments are proposed that will examine both the separate and the combined effects of alcohol and serotonin manipulations on two categorically distinct models of behavioral impulsivity (rapid-decision and reward-directed). To identify the time-course effects of these manipulations, healthy men and women will be evaluated at periodic intervals before and after pharmacological intervention. To determine whether rapid-decision and reward-directed models are differentially affected by alcohol and/or serotonin manipulations, each participant will experience all pharmacological conditions (repeated-measures design). In Experiment 1, we will examine dose effects of alcohol on behavioral impulsivity. In Experiment 2, we will examine dose effects of L-tryptophan loading and depletion (which alter central nervous system serotonin levels) on behavioral impulsivity. In Experiment 3, we will examine the interactive effects of alcohol and L-tryptophan manipulation on behavioral impulsivity. The aims of these studies are to determine: (1) the dose-dependent effects of alcohol (using 0.00, 0.25, 0.50, 0.75, and 1.00 g/kg of alcohol) on rapid-decision and reward-directed models of impulsivity;(2) the dose-dependent effects of L-tryptophan manipulation (using depleting, loading, and balanced amino acid drink mixtures) on rapid-decision and reward-directed models of impulsivity;(3) how a biological state change produced by L-tryptophan manipulation can moderate vulnerability to the behavioral effects of alcohol;(4) how different components of impulsivity (rapid-decision and reward-directed models) are differentially affected by the alcohol and L-tryptophan manipulations;and (5) how baseline responding on these behavioral models relates to one another (accounting for shared and unique variance) and to self-report measures of impulsivity. Together, these studies will yield information important to understanding the relationship between alcohol and L-tryptophan and their individual and combined effects on human impulsive behavior. This will provide evidence that serotonin may act as a moderating factor for alcohol-induced behavioral impulsivity. These studies will serve as a basis for further exploration of how the behavioral effects of alcohol impact underlying mechanisms of impulsivity and what factors contribute to the individual differences observed in impulsive behavior after alcohol consumption.