To elucidate the synergetic effects of acid rain on the development of pine wilt disease, we measured the sap¯ow rate in the stems and the chlorophyll content in the needles of 10-year-old Japanese black pine trees, Pinus thunbergii and 12-year-old Japanese red pine trees, Pinus densi¯ora, after exposure to simulated acid rain (SAR, pH 3) or tap water (TW, pH 6.3) as a control. The heat pulse method was used for the estimation of the sap¯ow rate. No apparent difference was found in the sap¯ow rate between the trees exposed to SAR and TW, but the chlorophyll content of needles at the end of the treatment was signi®cantly higher in the trees exposed to SAR than in those exposed to TW. When the pinewood nematode, Bursaphelenchus xylophilus, the causal agent of pine wilt disease, was inoculated onto the Japanese black pines that had been exposed to SAR repeatedly for 1 year, the period to death was shortened. Japanese red pines that had been exposed to SAR for 2 years, however, did not show any development of symptoms after the nematode inoculation, suggesting that acid rain only affects pine wilt disease slightly, if at all.