A: Xenograft volumes. From week 2 through 4 variations in xenograft volumes among treatment groups were significant (p < 0.0001 by Kruskal Wallis ANOVA). Gemcitabine (Gem) treatment significantly (p < 0.001 by Mann–Whitney test) reduced xenograft volumes in weeks 2 through 4 as compared with xenografts in untreated control mice. Simultaneous exposure of the animals to nicotine (Nic) in the drinking water significantly (p < 0.005 by Mann–Whitney test) reduced this effect of gemcitabine in weeks 2 and 3. Gaba (G) reversed this adverse effect of nicotine (p < 0.0005 by Mann–Whitney test) and reduced xenograft volumes below the sizes observed in animals treated with gemcitabine alone. GABA treatment alone was as effective as gemcitabine in reducing xenograft sizes. B: Xenograft volumes. From week 2 through 4 variations in xenograft volumes among treatment groups were significant (p < 0.001 by Kruskal Wallis ANOVA). Baclofen failed to reverse nicotine-induced gemcitabine resistance and did not significantly reduce xenograft volumes when administered as a single agent. Data points in graphs A and B are mean values and standard errors of xenograft volumes from 10 mice per treatment group.