Bioassay data obtained by the rat mesoappendix method lend themselves poorly to statistical representation. Sufficient reproducibility for quantitative estimation of the titer of vasoactive materials was achieved only with crystalline substances or purified biological extracts. Neither the duration of the displacement of epinephrine reactivity nor the magnitude of the shift in effective constrictor doses of epinephrine provided a satisfactory measure of vasoactivity in blood specimens. A prozone phenomenon, with agents such as Pitressin, histamine and ferritin was encountered with potentiating effects being elicited in high dose ranges and inhibitory effects in low dose ranges.