Physiologically active papaverine could not be recovered from the urine of humans after oral or parenteral admn. Instead, there appeared in the urine a probable derivative(s) which gave a quantitative turbidity response similar to papaverine with trichloracetic and phosphomolybdic acids. By comparing the cumulative ratio of urinary derivative output to papaverine intake with time, an equation for a hyperbolic curve was derived to fit most of the exptl. data. With this equation the metabolic half time for papaverine in man was estimated to be approx. 19, 14 and 7 hrs. after intramusc., intraven. and oral admn., respectively. The metabolic half time was significantly less for oral than for parenteral admn.