Plasma heparin concentrations were measured at two-hourly intervals following the subcutaneous injection of varying the doses of heparin. With doses of 5000 and 10,000 units, peak plasma concentrations were seen most commonly four hours after the injection. There was a wide range of peak values at both dose levels with considerable overlap and no correlation could be shown between plasma heparin concentration and the age, sex or weight of the individual. Traces only of heparin could be detected in the blood after a 1000 unit injection. Comparable results were obtained up to six hours after the injection with either the calcium or sodium salts of heparin.