Carotenoids comprise one of the most widespread classes of pigments found in nature. Polyene pigments from the dinoflagellate Gonyaulax polyedra were extracted every hour over a 24 hour period and the levels of beta-carotene during the day-phase were found to be twice as high as during the night-phase period. Using the thermal decomposition of 1,4-dimethylnaphthalene endoperoxide, the overall quenching ability of beta-carotene and G. polyedra carotenoids of singlet oxygen were found to be 2.04 x 10(9) M-1 s-1 and 4.75 x 10(9) M-1 s-1 respectively. Such a high quenching rate constant suggest a possible role for carotenoids as effective singlet oxygen quenchers in G. polyedra. The dose response curve for light-induced carotenoid synthesis shows a linear response up to 45 minutes of light exposure, after which night phase cells contain the same levels of beta-carotene as day-phase cells. Cells exposed to light pulses at different times during the night period display the highest beta-carotene induction in the middle of the night.