The effect of cooling on platelet-induced contractions was studied. Rings of canine saphenous arteries were suspended for isometric tension recording in organ baths filled with aerated physiological salt solution. Norepinephrine, 5-hydroxytryptamine, and autologous aggregating platelets all caused contractions that were augmented by cooling the bath content from 37 to 24 degrees C. These contractions were inhibited, in a concentration-dependent manner, by the serotonergic antagonist ketanserin. The alpha 1-adrenergic antagonist, prazosin, in concentrations causing progressive inhibition of contractions evoked by norepinephrine did not affect the response to either 5-hydroxytryptamine or platelets. Aggregating platelets were found to release 5-hydroxytryptamine in sufficient amounts to account for the observed contractions. Pretreatment of platelets with the cyclo-oxygenase inhibitor meclofenamate reduced the amount of thromboxane liberated by aggregating platelets but did not influence evoked contractions. These observations suggest that aggregating platelets cause contraction of the canine saphenous artery by releasing 5-hydroxytryptamine. They demonstrate that cooling markedly augments contractions of peripheral arterial smooth muscle caused by aggregating platelets.