Vitamin A and P-carotene concentrations in serum, in fluid of follicular cysts and in variablysized follicles were determined, Contrary to p-carotene, which showed no change in concentration, vitamin A increased significantly during follicular growth from 18 l g / 100 ml in the small to 28 pg / 100 ml in the large follicles. This increase is assumed to be the result of a conversion of p-carotene to' vitamin A by a carotene-cleaving enzyme. Since this increase in vitamin A occurs in only one follicle per cow, it may well be of importance for oocyte maturation. An essential local provitamin A function of @-carotene in bovine fertility may therefore be postulated.

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