Abstract: The effects of nucleic acid antimetabolite on photoperiod-sensitive genic male sterility in rice (PGMR) were studied by the application of 5-fluorouracil (5-FU) to leaves. The results showed that 5-FU treatments decreased the panicle length and the spikelet number of Nongken 58S, but hastened heading and to some extent recoveried the fertility of PGMR under LD conditions; whereas 5-FU treatments inhibited the pollen fertility and the heading of rice varieties under SD conditions. The effects can be partially reversed by oratic acid. It can be concluded that photoperiod-sensitive genic male-sterility is caused by the changes of the metabolism of nucleic acid in young panicle.