Studies on the synthesis of previtamin D₃(PreD₃), lumisterol₃(L₃) and vitamin D₃(D₃) in dorsal skin of broiler chicks irradiated by ultraviolet(U.V.) light of different wavelength were carried out. The dorsal skin of 3 week-old chicks fed vitamin D-defficient diet were exposed to U.V. light of 254, 310 or 365nm for 0, 30, 60, 120 or 150 minutes. The vitamin D₃ and its analogues were extracted from epidermis of dorsal skin and analyzed by HPLC. 1. When the dorsal skin of 3 week-old chicks were irradiated by 254nm UN light, the 7-DHC contents were decreased continuosly from initial 255 to 55ng/㎠ for 150 min exposure; the decreasing amount was 200ng/㎠ and the decreasing rate was 1.3ng/㎠/min. The L₃ contents were increased from first 110 to 115ng/㎠ of 150min. irradiation ; 5ng/㎠ was synthesized, being the rate of 0.03ng/㎠/min. The PreD₃ contents were decreased from 58 to 5ng/㎠ of 150min. exposure; 53ng/㎠ of PreD₃ was decreased at the rate of 0.14ng/㎠/min. The D₃ contents were increased from original 2 to 30ng/㎠ of 150 min. irradiation ; 28ng of D₃ was produced and the production rate was 0.19ng/㎠/min. 2. As the dorsal skin of 3 week-old chicks were exposed to the 310nm U.V. light, the 7-DHC contents were decreased from initial 340 to 27ng/㎠ for 150mm. exposure ; 313ng/㎠ was disappeared at the rate of 2.1ng/㎠/min. The L₃ contents were increased from original 170 to 400ng of 60 min. irradiation ; 230ng/㎠ of L₃ was synthesized at the rate of 3.8ng/㎠/min. The D₃ contents were increased from 85 to 320ng/㎠ of 90 min. irradiation ; 235ng/㎠ of D₃ was synthesized at the rate of 2.6ng/㎠ min. The PreD₃ contents were changed from 52 to 70ng/㎠ of 60min. treatment ; 18ng/㎠ was produced at the rate of 0.3ng/㎠/min. 3. When the dorsal skin of 3 week-old chicks were irradiated by 365nm U.V. light, the 7-DHC consents were diminished from 800 to 200ng/㎠ by 150 min. exposure ; 600ng/㎠ was disappeared at the rate of 4ng/㎠/min. The PreD₃ contents were increased from initial 40 to 75ng/㎠ by 60 min. irradiation ; 35ng/㎠ were synthesized at the rate of 0.6ng/㎠/min. The L3 contents were increased from original 12 to 15ng/㎠ of 30 min. exposure ; 3ng/㎠ was produced at the rate of 0.1ng/㎠/min. The D₃ contents were increased from initial 0 to 50ng/㎠ of 90 min. exposure ; 50ng/㎠ was generated at the rate of 0.56ng/㎠/min. 4. It was verified that 310nm U.V. light would be desirable to irradiate the dorsal skin of 3 week-old broiler chicks rather than 254 or 365 nm lamp in order to sythesize D₃ and its analogues.