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Three experiments were conducted to determine (1) the pharmacodynamics of 5-hydroxytryptamine in juvenile lobsters; (2) the effects of 5-hydroxytryptamine, using a range of dosages, on a motor behavior used to escape an aversive situation; and (3) the effect of doses that did and did not inhibit this motor behavior on measures of dominance and shelter competition. The fate of 5-hydroxytryptamine in hemolymph over a 60-min post-injection period showed that the concentration fell rapidly to a low plateau that was maintained for at least 1 h. Low doses of 5-hydroxytryptamine did not affect locomotor behavior, but higher doses inhibited it. Dominance and subsequent possession of a shelter were unaffected by a low dose of 5-hydroxytryptamine but a higher dose that inhibited locomotion resulted in lobsters that lost fights and did not secure or retain possession of the shelter. In the context of dominance and shelter competition, we were unable to demonstrate any advantage of the low dose of exogenous 5-hydroxytryptamine and a severe disadvantage with the higher dose. Previous reports of transient increases in aggression in 5-hydroxytryptamine-treated subordinate lobsters did not take into account motor inhibition as a possible critical variable in aggression.