[en] The photochemical conversion of 7-dehydrocholesterol (7-DHC) into vitamin D₃ in rat skin was experimentally studied. The skin stripped off from a sacrificed normal rat was irradiated with an ultraviolet light for a constant period in the first in vitro experiment. The normal rat irradiated under the same conditions mentioned above was sacrificed and then the skin was stripped off in the second in vivo experiment. Lipids were individually extracted with chloroformmethanol (1:1) from the skin obtained in the two experiments and the solvent was evaporated. The resulting residue was saponified and the unsaponified matter extracted with benzene was purified by application to hydroxyalkoxypropyl (HAP) Sephadex column chromatography. The resulting purified vitamin D₃ fraction was applied to high performance liquid chromatography (HPLC) in order to estimate vitamin D₃. No peak, aside from that of alphanaphthol as an initial standard, was observed in the HPLC chromatogram on the skin obtained from the non-irradiated rat, whereas the peak corresponding to vitamin D₃ was observed in each HPLC chromatogram on both the irradiated skin (in vitro experiment) and the skin obtained from the irradiated rat (in vivo experiment). The result shows that 7-DHC in rat skin was photochemically converted into vitamin D₃. (Iwakiri, K.)