The establishment of MACC (Ceiling Values) and MACT (Time Weighted Average Values) for chemical substances permits a substantial more precise evaluation of the exposure to harmful substances at the work place as if only one limit value is given without reference to a Ceiling Value or a Time Weighted Average Value, respectively. The derivation of MACC- and MACT-Values is proposed from toxicometric data that are gained during animal experiments with chemical substances. For acutely toxic substances (e.g. irritants) the quotient F = MACC/MACT ought to be = 1. For cumulative acting substances the permissible relation of both values is determined from the magnitude of the zone of chronic effect (Zch) as F = 0.4. Zch. The condition MACC less than less than Limac takes into consideration at the same time. The frequency and the duration of MACT exceedings need not to be regulated with this procedure. A method is proposed for the derivation of the length of sampling periods necessary from the toxicological point of view starting from the Zch.