InhaltDie intramuskuläre Injektion des Depotgestagens Chlormadinonacetat in Kühe des 5. Zyklustages führte zu einer vorzeitigen Regression des periodischen Gelbkörpers. Corpora lutea coin 14. – 16. Zyklustag wares bei behandelten Kühen leichter, zeigten schwächere Steroiddehydrogenase‐Aktivität, einen geringeren Anteil aktiver Luteinzellen in einem aufgelockerten Zellverband. Diese histologischen Veränderungen entsprachen Strukturen, wie sic in regressierenden Gelbkörpern unbehandelter Kühe gefunden werden.Contents(Premature regression of the Corpus luteum in cows following an injection of progestagen during early dioestrus)The i.m. injection of 30 mg Chlormadinonacetate (CAP) in cows on day 5 of their sexual cycle resulted in a premature regression of the corpus luteum. In treated animals C.L. of day 14 to 16 of the sexual cycle were 20 % lighter in weight, had less intensity of steroid dehydrogenase activity and showed fewer and disorderly distributed lutein cells. These histological and histochemical findings are similar to structures found in normal r