SummaryChanges in levels of nicotinamide adenine nucleotides were measured during a short (30 min) period of illumination following a dark period. Two phases in the time course were found. In the first phase, during the first minute of illumination, a rapid decline in oxidized nicotinamide adenine dinucleotide occurred which represented a net loss of nicotinamide adenine nucleotide. In the subsequent, second phase during illumination, a slower decline in oxidized nicotinamide adenine dinucleotide was found which was coincident with increases in nicotinamide adenine dinucleotide phosphate(s). The changes in the reduced nicotinamide adenine nucleo· tides were relatively small during illumination.Changes of 14C in intermediates of the Krebs cycle and related compounds, labelled with 14C during a pre· incubation period in the dark in the presence of 14C02, were correlated with the changes in nicotinamide adenine nucleotides. Again two phases in the time course were observed. In the first minute of illumination a rapid decrease of 14C in aspartate coincided with an equivalent increase in citrate. Sub· sequently, an apparent conversion of aspartate to malate was found.These changes are discussed in relation to the effect of illumination on the ratio of oxidized to reduced nicotinamide adenine dinucleotides and Krebs cycle metabolism.