Light-sensitivity of the first and the last processes in the inductive dark period, and the effect of twilight on photoperiodic induction were investigated with four short day plants. In Oryza sativa (Akebono), the first and the last processes are relatively light-insensitive, and the photoperiodically effective day length under natural conditions is considered to be equal to the astronomical day length. In Biloxi soybean and Perilla frutescens, the first process is re latively light-insensitive, but the last process is more sensitive. Effective dark period seems to begin when twilight is 50∼200 lux in the evening and continue until it is about 10 lux in the morning. In Xanthium saccharatum, both the first and the last processes are relatively light-sensitive, but the former is more sensitive than the latter. Effective dark period seems to begin when twilight is 1∼10 lux in the evening and continue until it is 10∼50 lux in the morning.