Because the leaf water potential (Ψ1) of the rice plant (Oryza sativa L.) is strongly affected by the evaporative demand of the atmosphere under paddy field conditions, and most changes in Ψ1 reflect variations in evaporative conditions, we can not directly compare seasonal trends of Ψ1 in terms of phenological changes of water status of the plant. Our objective was to determine whether the ability to maintain high Ψ1 under paddy field conditions changes with growth stage independent of atmospheric conditions. Maintenance ability was determined by calculating the slope of the regression line between the evaporation rate from a wet filter paper (E0) and Ψ1 over a 24 hour period. This technique was chosen because there were close relationships between E0 and Ψ1 within a day, and the slope of the regression line (S) can be considered to indicate the sensitivity of Ψ1 to the evaporative demand of the atmosphere. A three year experiment was conducted at two locations in East Shimane Prefecture using three rice cultivars ; the two locations were Matsue, less than 17m above sea level, and Akana, at 444m. For the first group (Matsue), S was moderate between the stages of transplanting and formation of young panicles, but became significantly steeper during the grain filling stage. In the second group (Akana) S followed the same trend before the grain filling stage, but then continued to be maintained moderately until the end of the season-this applied both to the plants at Akana and to late planted samples at Matsue. Both of these latter sets of samples were subject to different growing conditions from the norm, especially lower temperatures during grain filling than what would be normal at Matsue. We concluded that maintenance ability of Ψ1 in rice plants changes with growth stage, but this ability may vary, particularly in the grain filling period, as a result of growing conditions.