Pre-harvest sprouting(PHS)in rice production is usually caused by high temperature and humidity or continuous rains.It frequently happens in F1 in hybrid rice seed production.The PHS or "Physiologically germinated" seeds are of lower quality,by which the hybrid rice seed production is badly affected every three years at a loss of 20% or even 50% yield in seed production over the vast Yangtse River Valley and Southwest China.It is estimated that PHS causes an average decrease of seed activity by 10%.A recombinant inbred line population including 304 lines,derived from a cross between Indica rice(Oryza sativa L.)cultivars Zhong-156 and Gumei-2,was used to study the PHS physiology.Based on the data of sprouting rate in panicles and sprouting rate in grains,two kinds of lines,namely easy-to-sprout lines and hard-to-sprout lines,were selected to investigate their physiological differences when PHS happened.The experiment was conducted in a special field with a microclimate of higher temperature and humidity.The results indicated that it was easier to produce PHS from the female parent GM-2 than the male parent ZH-156,besides,the GA1 content and amylase activity in GM-2 grains were higher than those in ZH-156.However,the abscisic acid(ABA)content in GM-2 grains was lower than that in ZH-156.Higher temperature and higher humidity facilitated the GA~ increment from milk ripe stage to yellow ripe stage.GM-2 and the easily-sprouting lines showed an even higher increase in GA1 than ZH-156 and the hardly-sprouting lines,which enhanced the amylase activity and induced pre-harvest sprouting.This may be the physiological basis for pre-harvest sprouting induced by higher temperature and higher humidity,and these special characteristics must be inherited from their parents.