CERTAIN types of hazard consequent on the administration of chemical substances are estimated by the performance of chronic toxicity tests. Such tests are usually designed according to certain empirical rules. Groups of animals (commonly three) are given a test substance in regularly repeated doses of constant size, each group receiving a different dose; the material can be given either by individual dosing to each animal or by administration in the diet. The interval between doses in the case of individual doses may vary according to the properties of the substance; the most generally used interval is 24 h. The experiment continues for an arbitrarily chosen period of time since no general principle enables the duration of such tests to be determined more rationally.