Taichung (Native) No. 1 rice was grown under controlled conditions with a basic temperature regime of 30°C by day and 20°C by night. Periods of 2, 4 and 12 hours at 15°C (59°F) were imposed during darkness for the first or second 40 days, or for the first 80 days after sowing. Floral initiation was delayed by cool periods of 2 hours at night or longer, and delay from this cause was always accompanied by reduced grain yields, resulting from fewer spikelets per panicle or increased tiller sterility. This effect was not overcome when the cool periods ceased after floral initiation. Cool periods longer than 4 hours had a greater effect when they were given in the first 40 days.