Inhibitory effect of the following nine proteinase inhibitors on the renin activity was studied. These were four kinds of trypsin inhibitors, Trasylol, pepstatin A, chymostatin, antipain and leupeptin. Except pepstatin A, none of these inhibited renin in vitro. The type of inhibition of renin by pepstatin A was competitive. Ki value for pepstatin A was found to be 3.2 × 10^-7M.