Release of platelet-free plasma serotonin is effected by intravenous endotoxin injection in rabbits. Maximal elevation of serotonin is present 15 seconds after endotoxin and the magnitude of elevation of plasma serotonin is correlated with the dose of endotoxin. There was no significant rise of plasma serotonin with a non lethal dose. These changes are concurrent with thrombo -cytopenia and a decrease in serum serotonin, and correlate well with those observed in anaphylaxis.