Effects of light intensity and irradiation time on respiration rate in the dark period after irradiating are discussed. The specific respiration rate of S. platensis in the dark is a constant value Q(1), up to 15-30 minutes, and then rapidly decreases to Q(2) after 30 minutes. The specific respiration rate Q(1) increases with light intensity. No significant effect of Q(1) on irradiation time and Q(2) on light intensity is observed. The respiration rate for a long dark time, Q(0) (=1.2 mg-O-2.kg-Spirulina(-1).s(-1)), is almost the same as Q(2). The effect of the logarithmic mean light intensity I-1m[klx] on Q(1) is described by the following equation. Q(1)-Q(0) = 13 I-1m/15+I-1m.