The noradrenaline content of the heart and spleen was investigated in normal mice and in mice treated with drugs. A modification of the methods of Bertler, Carlsson & Rosengren (1958) was used for extraction, and of v. Euler & Floding (1955) for fluorimetric estimation of the amine. In normal mice the mean noradrenaline content of the heart was 0.55 mug/g and that of the spleen 0.26 mug/g fresh tissue. Iproniazid (100 mg/kg), nicotine (0.1 mg/kg) and histamine (0.5 mg/kg), given 1 and 3 hr before killing the mice, did not significantly change the concentration of noradrenaline in the heart. Neither did nicotine and histamine, administered 1 hr before death, significantly alter the noradrenaline content of the spleen. The rapid changes in the catechol amine content of mouse tissues reported with these drugs by De Schaepdryver & Preziosi (1959) were not observed. In contrast, reserpine (2.5 mg/kg), methyl reserpate methyl ether (1 mg/kg), and methyl 18-epireserpate methyl ether (2 mg/kg) caused severe depletion of noradrenaline from the heart and spleen of the mice.