Cats (5), in which the aorta contained a 6 .mu.g injection of serotonin in a closed aortic segment, exhibited depressed hindlimb blood flow. The serotonin effects were eliminated in 3 animals by pretreatment with cinarserin HCl, a serotonin antagonist. The caudal 1.5 cm of the aorta was occluded in 24 cats by a blood clot formed by injection of thromboplastin subsequent to ligation. Of these animals, 8 were pretreated with cinanserin HCL and exhibited a significant improvement in hindlimb collateral blood flow. Serotonin was reduced in 9 of these animals with reserpine/p-CPA [p-chlorophenylalanine] treatment, and they exhibited the most significant recovery of all treated animals. Of the cats with blood serotonin reduced by decreasing blood platelets, 7 showed no improvement in hindlimb blood flow. Serotonin is apparently a factor in the inhibition of collateral blood flow which follows arterial thrombosis.