Using pot-grown Wase satsuma mandarin (Citrus unshiu Marc.) trees grafted on trifoliate orange (Poncirus trifoliata Raf.) and Natsudaidai (C. natsudaidai Hayata) seedlings, the effect of water stress on cold hardiness was examined by withholding water in varying periods of time from fall to winter. Freezing tests were conducted at intervals during the withholding of water to evaluate cold hardiness. Water relations and carbohydrate and proline contents in plant parts were also determined. Almost the same results were obtained with both species and are summarized as follows.1. Transpiration and leaf water potential of water-stressed trees were always lower than in non-stressed (irrigated) ones, though those characters in the non-stressed trees declined gradually from November to January.2. Freezing tests done in November and December revealed that water stress given beforehand increased cold hardiness. However, freezing tests in January, when even non-stressed trees were already hardened sufficiently by natural low temperature, demonstrated that the trees stressed from October or November on were less hardy compared to non-stressed controls, though the trees stressed from December on were somewhat hardier.3. Water stress increased proline contents in leaves, stems and roots while it reduced starch contents. Total sugar contents in November and December in the trees stressed earlier were higher than in non-stressed ones. However in January, sugar contents in the leaves and stems of the trees stressed from October or November on were lower than in the trees non-stressed or stressed from December on. These sugar changes were closely related to the changes in cold hardiness.4. In conclusion, it is suggested that under relatively warm temperature conditions from fall to early winter, water stress induces cold hardening, but that in mid-winter when the trees have been naturally hardened, any beneficial effect of water stress given beforehand is diminished or even becomes detrimental.