The effects on primary haemostasis of unfractionated heparin and of the two low molecular weight heparins, enoxaparin and fragmin, were compared in two rat models, one employing the gastric mucosa and the other the tail skin. All three heparin preparations prolonged the bleeding time and increased the blood loss dose dependently. The prolongation of the bleeding time per unit dose caused by unfractionated heparin was significantly greater than the prolongation caused by either one of the two low molecular weight heparins. In the gastric mucosa, but not in the tail skin, enoxaparin prolonged the bleeding time significantly less than fragmin (p < 0.05).