The tolerance of ethyl (Z)-(3-methyl-4-oxo-5-piperidino-thiazolidin-2-ylidene)acetate (etozolin, Gö 687, Elkapin) has been investigated over 18 months after oral administration in rats and over 1i months after intragastric application in dogs. Further the influence of the compound on pregnant animals and their fetuses after intragastric application has been investigated in rats and rabbits. The investigation of the influence on fertility and breeding capacity as well as on the peri- and postnatal development was carried out in rats after peroral administration. In the course of the chronic experiments a dose related increase of the excretion of fluid and electrolytes was observed. Side-effects, which mainly occurred during the last weeks of the experiment in the highest dosage group, were most probably due to the exhaustion of the fluid and especially of the electrolyte reserves of the animals. The experiments in rats and rabbits did not yield any findings indicating teratogenic properties. Fertility and breeding capacity were likewise not influenced by etozolin. An influence on the peri- and postnatal development could be demonstrated only in the toxic dose range.