Embryonic chick notochords were studied during their metabolically active and involuting periods for the expression of collagen type I and II. The staining was carried out on notochordsin vivoat stage 20 and stage 35 and on mesenchyme‐contaminated and mesenchyme‐free notochords at stage 20, which were culturedin vitrofor 6 days. The results show that type II collagen is demonstrable in the notochords, at all the examined stages, bothin vivoandin vitro. However, the expression of type I collagen was stage‐dependentin vivoandin vitro. At stage 20, the perinotochordal sheath is positively immunostained for collagen type I, but the notochord itself is negative. At stage 35, the perinotochordal sheath as well as the notochord are positively immunostained for collagen type I. The mesenchyme‐contaminated and the mesenchyme‐free notochords and their sheaths are also positively immunostained for the type I collagen after6 daysin vitro. The current results, at late developmental stages, indicate that the involuting notochords express collagen type I, which seems not to be altered by changing the micro‐environ
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