A cDNA for delta1-pyrroline-5-carboxylate (P5C) synthetase (cOsP5CS), an enzyme involved in the biosynthesis of proline, was isolated and characterized from a cDNA library prepared from 14-day-old seedlings of Oryza sativa cv. Akibare. The deduced amino acid sequence of the P5CS protein (OsP5CS) from O. sativa exhibited 74.2% and 75.5% homology to that of the P5CS from Arabidopsis thaliana and Vigna aconitifolia, respectively. Northern blot analysis revealed that the gene for P5CS (OsP5CS) was induced by high salt, dehydration, treatment of ABA and cold treatment, while it was not induced by heat treatment. Simultaneously, accumulation of proline was observed as a result of high salt treatment in O. sativa. Moreover, the levels of expression of OsP5CS mRNA and content of proline under salt stress condition were compared between a salt-tolerant cultivar, Dee-gee-woo-gen (DGWG) and a salt-sensitive breeding line, IR28. It was observed that the expression of the P5CS gene and the accumulation of proline in DGWG steadily increased, whereas those in IR28 increased slightly.