The compatibility of ethyl (Z)-(3-ethyl-4-oxo-5-piperidino-thiazolidin-2-ylidene)acetate (piprozoline, Gö 919, Probilin) was tested orally in rats and intragastrically in dogs for 6 months. Moreover, the effect of the substance was studied on pregnant rats and rabbits and their foetuses after intragastric administration and on the fertility and breeding capacity of rats after oral application. In the course of the chronic experiments, in the highest dosage group of rats a slight sedation and a decrease of body weight gain and food intake could be observed. The female animals showed an enlargement of the adrenal glands. A small hyperemesis was noted in the highest dosage group of dogs. Subtoxic doses applied to rats and rabbits during gestation did not induce malformations in the newborns. Even treatment with large doses did not influence fertility and breeding capacity.