Time versus concentration curves of unchanged protacine were measured in the blood of 5 healthy volunteers by high pressure liquid chromatography after a single oral dose of 4 capsules (600 mg). These curves showed a maximum at 8 hours after administration, with an absorption half-life of 3.4 hours, and were interpreted according to the two-compartment open model with invasion. The elimination half-life was found to be 10 hours. This may allow a maintenance treatment, where necessary, with 1 capsule (150 mg) of protacine twice daily and strongly suggests that, apart from acute cases, protacine would be particularly well suited for long-term treatment.