Methylergometrine concentrations in human and rabbit plasma were determined by a new radioimmunoassay after a single intravenous injection (0.2 mg in man and 0.05, 0.1 and 0.2 mg/kg in rabbits). Both in man and in the rabbit methylergometrine disappeared quickly from the plasma with a mean T1/2alpha of 1.8 and 1.2-1.7 min. respectively. Similarly, the T1/2beta-values were 32.1 and 27.3-93.2 min. tthe mean maximal response in the rabbit uterus in situ after 0.05, 0.1 and 0.2 mg/kg intravenously dose was found at 40 sec., 26 sec., and 26 sec. after the drug administration, respectively, and the dose response curve was quite steep. A significant correlation was found between the dose and response.