1. The effects of 5-hydroxytryptamine (5-HT) in the absence and presence of noradrenaline (NA) or the thromboxane-A2 mimetic, U44069, were investigated in ring preparations of marmoset aorta. 2. 5-HT (0.001-10 mumol/L) produced little or no contractile response in preparations at basal tone. When the tone was elevated to 50% of maximum with NA the predominant response to 5-HT was relaxation. The 5-HT2 receptor antagonist LY53857 (0.1 mumol/L) unmasked a contractile response to low concentrations of 5-HT (0.01-1.0 mumol/L) and reduced relaxation to high concentrations of 5-HT (1.0-10 mumol/L) in vessels precontracted with NA. 3. In U44069-contracted vessels, 5-HT was contractile in the range 0.01-1 mumol/L and relaxant in concentrations of 6.0-10.0 mumol/L. Ketanserin (1.0 mumol/L) had no effect on the contraction or relaxation to 5-HT. 4. The relaxant response to 5-HT was not significantly diminished in endothelium-impaired arteries. 5. In conclusion, 5-HT exerts complex inhibitory and excitatory actions on the marmoset aorta. The inhibitory actions are not endothelium-dependent and the excitatory actions do not appear to involve the 5-HT2 receptor.