The distribution of radioactivity in pregnant mice was registered at 0, 4, and 24 hrs after a 10 min. period of inhalation of 14C-halothane. Autoradiographic methods were used to allow to distinguish between the distribution of volatile (non-metabolize) halothane, water-soluble metabolites, and firmly tissue-bound metabolites. While volatile radioactivity was seen predominantly at short survival intervals, e.g. in body fat, blood, brain and liver, metabolites accumulated with time. Peak values occurred at 4 hrs in most organs (measured with liquid scintillation as well). The most remarkable findings were the high concentrations of radioactivity in amniotic fluid (and the ocular fluids of adults) with peak values at 4 hrs and rather high concentrations still prevailing at 24 hrs after inhalation. It is assumed that this activity represents only partly volaile halothane and mostly non-volatile metabolites. High activity of metabolites was seen in the neuroepithelium of the embryo in early gestation. Firmly tissue-bound metabolites, still remaining after washing the tissues with trichloroacetic acid and organic solvents, were found in the nasal mucosa, trachea and bronchial tree and in (presumably centrilobular) zones of the liver of adults after inhalation and 5-day old mice after intraperitoneal injection, indicating the formation of reactive metabolites in these organs. Firmly tissue-bound activity was not observed in the corresponding foetal organs.