Full-thickness pieces of adult rat trachea were supported on rayon on the surface of clotted medium in watch glasses. Differentiated epithelium was reduced in height during 25 days of cultivation because basal cells and some columnar cells migrated to cover exposed parts of the explants and because some differentiated cells died and were shed.

S35-methionine was (a) placed on explants in vitro and (b) injected intraperitoneally in living rats. Cultured tissues and tissues of living rats were examined by autoradiography at 4 and 24 hours and 4, 7, and 11 days after labeling.

Although migratory undifferentiated epithelial cells appeared in cultured trachea, all living epithelial cells in vitro incorporated and subsequently lost S35-methionine to the same extent as did epithelium of intact rats. The biologic half-life of methionine in rat tracheal epithelium in vivo and in vitro was about 5 days.

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