Large changes in the rate of synthesis of UDP-apiose synthase, an enzyme of the flavonoid glycoside pathway, were observed both in vivo and in vitro following irradiation of previously dark-grown cell suspension cultures of parsley (Petroselinum hortense). Irradiation of the cells with ultraviolet light for 2 1/2 h caused a large increase in UDP-adipose synthase mRNA activity which continued for several hours during the subsequent dark period. A sharp peak in this activity was followed by a rapid exponential decline. The timing of changes in mRNA activity was very similar for UDP-apiose synthase and chalcone synthase, a previously investigated enzyme of the flavonoid glycoside pathway, but differed from that for phenylalanine ammonia-lyase, an enzyme of general phenylpropanoid metabolism. Thus, the present data provide further evidence for a differential regulation of the two groups of enzymes and a high degree of co-ordination within each group. The expected light-induced changes in UDP-apiose synthase activity were calculated from the observed changes in the rate of synthesis, allowing for an apparently constant rate of degradation (t 1/2 = 17 h). The resulting time course was in agreement with experimental data, suggesting that the changes in enzyme activity were caused by corresponding changes in the rate of synthesis. A protein of unknown function (Protein X), which is tightly associated with UDP-apiose synthase and co-purifies with the enzyme at a molar ratio of 1:1, is shown to be regulated independently of the enzymes of the flavonoid glycoside pathway.