N,N-Dimethylcarbamoylmethyl-4-(4-guanidino-benzoyloxy)phenylacetat e methanesulfonate (camostat mesilate) is reported to be an effective inhibitor of plasma kallikrein. It was shown in vitro to inhibit not only plasma kallikrein, but also renal kallikrein and plasma kininase II. These inhibitory activities, however, were very weak. The inhibition of plasma kallikrein in human plasma was limited in time, since a rapid reactivation of plasma kallikrein was noticed when samples were incubated at room temperature. In order to establish whether camostat mesilate was able to inhibit plasma kallikrein, kininase II and renal kallikrein also in vivo the inhibitory activity of camostat mesilate on these enzymes was studied in 5 healthy volunteers. After an oral intake of a single dose of 600 mg of camostat mesilate, plasma kallikrein was inhibited significantly, while kininase II in plasma was unaffected. Renal kallikrein activity determined by urinary excretion of active kallikrein remained unchanged after camostat mesilate intake. Thus, the results demonstrate that camostat mesilate in vivo inhibits only plasma kallikrein and has no effect on the activity of kininase II or renal kallikrein.