For the first time 7-dehydrocholesterol (7-DHC, provitamin D-3) photoisomerization was studied in nematic LC matrices (ZLI-1695, ZhK-805) in comparison with ethanol solution using UV absorption spectroscopy. Dissolution of chiral 7-DHC molecules in the nematic LCs induces cholesteric phase, and dramatic effect of the cholesteric pitch value on the spectral kinetics has been revealed. Significant increase in trans-isomer accumulation was observed as the cholesteric pitch was reduced from 2200 to 25 mkm. Noticeable effect of the LC orientation has been observed as well.