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IN the dog, heparin and histamine are curiously related. Thus the various forms of histamine shock in the dog, whether elicited by peptone, by a " shock dose " of protein or by pure chemical histamineliberators (MacIntosh and Paton, 1949), are all accompanied by incoagulability of the blood. The main source of both the heparin and the histamine is thought to be the liver, the " shock organ " of this species (Rocha e Silva, 1952), and the heparin moiety is generally attributed to the mast cells which are abundant in the liver of the dog (Wilander, 1939). The appa,rent absence of heparin in states of shock in other animal species rather indicates that in the dog the association of heparin and histamine is fortuitous, but the application of histamine-liberators to the rat described in the present paper suggests that in this species at least there may well be a closer relationship than has as yet been realised and that histamine as well as heparin may come from tissue mast cells. MATERUS AND METHODS Experimental animals. Adult male and female rats of the Wistar strain, wt. 175-225 g., were used.Solutions for injection. The histamine-li berators and the concentrations i n which they were given by intravenous injection are shown in the accompanying table.