The purpose of the present work was to examine gene expression patterns in rat skin exposed to a beam of 56Fe ions, a surrogate for the high-energy, heavy-ion galactic radiation background, as a basis for obtaining a better understanding of the possible mechanism(s) behind the radioprotective activity of vitamin A. A 2 × 4-cm rectangle of dorsal rat skin was exposed to 1.01 GeV/nucleon 56Fe ions generated by the Alternating Gradient Synchrotron at Brookhaven National Laboratory. Gene expression patterns were monitored in either the presence or absence of a 250-ppm dietary supplement of vitamin A acetate in powdered lab chow. Although vitamin A and other retinoids show anti-carcinogenic activity in several animal models, the underlying changes in gene expression have not been examined extensively. At either 1 or 7 day after irradiation, a 1-cm square of irradiated and control rat skin was excised and analyzed using the Affymetrix rat microarray (RG-U34A) system. Microarray responses were displayed and processed by GeneSpring 7.0 and GOTree software. At 1 day after 3 Gy of 56Fe-ion irradiation, the expression of 110 genes was significantly up-regulated (P < = 0.05) in comparison to levels in control rat skin, while no genes were altered by the vitamin A acetate supplement alone. Combined with 56Fe-ion radiation, the vitamin A acetate supplement blocked the expression of 88 (80%) of the 110 genes and eliminated 16 of 18 gene categories that were significantly altered (all increased) by the 56Fe-ion radiation. Categories with large numbers of genes eliminated by the retinoid included response to stress, 33 genes; response to biotic stimulus, 38 genes; signal transduction, 35 genes; and regulation of cellular/physiological process, 40 genes. Even for immune response and response to biotic stimulus, the only two categories that remained significantly altered in the presence of the vitamin, the combined number of altered genes was reduced from 74 to 13. No significant alterations in gene expression were found at 7 days relative to the numbers in controls. The results indicate that at 1 day dietary vitamin A acetate strongly interfered with 56Fe-ion-induced gene expression within the broad categories of stimulus- and stress-related genes, implying that the latter gene categories likely play a role in the radioprotective action of the vitamin.