The method of rebreathing and its particular importance to the estimation of blood alcohol concentration (BAC) are described. A face mask with an accompanying valve chamber to which a collection bag was attached proved to meet the requirements arising from the physiological exchange processes in rebreathed air. Practical tests in the laboratory revealed excellent agreement between the values obtained by direct measurements of breath alcohol by means of an IR-Spectrophotometer and those from stored samples. The application of the method described is of particular importance in cases of unconscious and benumbed patients, of sleeping and uncooperative patients and those with collapsed veins suspected of heavy alcohol intoxication. It has been proven to be an essential aid in differential diagnosis since it allows instantaneous determination and continuous monitoring of the BAC-level which aids in the observation of the reduction of this level. This was demonstrated in practice with 9 patients at the alcohol emergency clinic in Mainz on the carnival "Rosenmontag".