1. 5-Hydroxytryptamine (5-HT) induced dose-dependent contractions in the isolated cat femoral artery, which were reduced by LSD, methysergide, phentolamine and reserpine pretreatment (only at low doses). 2. Pentobarbital (PB) and Mn/+ relaxed the arteries previously contracted with 5-HT. These drugs reduced the contraction evoked by this amine as it was Ca2+-suppression. 3. High concentrations of 5-HT and K+ induced tritium release from vessels prelabelled with 3H-noradrenaline. Ca2+-deprivation and PB unmodified the release caused by 5-HT, but that elicited by K+ was abolished. 4. These data indicate that 5-HT-induced contraction is essentially due to direct interaction of this agent with 5-HT-receptors, and that PB interferes with Ca2+ entry to the cell.