The 80% aqueous acetone extract from the rhizomes of Alpinia officinarum, a Chinese medicinal herb, were found to inhibit melanogenesis in theophylline-stimulated murine B16 melanoma 4A5 cells. Among the constituents isolated, four diarylheptanoids [5-hydroxy-1,7-diphenyl-3-heptanone, 7-(4('')-hydroxy-3('')-methoxyphenyl)-1-phenylhept-4-en-3-one, 5-hydroxy-7-(4('')-hydroxy-3('')-methoxyphenyl)-1-phenyl-3-heptanone, and 3,5-dihydroxy-1,7-diphenylheptane] and two flavonol constituents (kaempferide and galangin) inhibited melanogenesis with IC(50) values of 10-48microM, and several structural requirements of the active constituents for the inhibition were clarified. In addition, 7-(4('')-hydroxy-3('')-methoxyphenyl)-1-phenylhept-4-en-3-one, kaempferide, and galangin inhibited mRNA expression of tyrosinase and tyrosinase-related proteins-1 and -2, and the protein level of a microphthalmia-associated transcription factor.