Irradiation of previously dark-grown cell suspension cultures from parsley (Petroselinum hortense Hoffm.) with ultraviolet light caused large, concomitant increases in mRNA activities for two characteristic enzymes of phenylpropanoid metabolism, phenylalanine ammonia-lyase, and flavanone synthase. The rates of enzyme synthesis both in vitro in a reticulocyte lysate and in vivo were quantitated by direct immunoprecipitation of the labeled enzyme subunits. Following a period of about 2 h, during which possible changes were below the limits of detectability, the two mRNA activities increased rapidly in irradiated cells for serveral hours. Phenylalanine ammonia-lyase mRNA reached a peak in activity a few hours earlier than flavanone synthase mRNA. The apparent half-lives of the enzyme activities were about 7 to 10 h for phenylalanine ammonia-lyase and 5 to 7 h for flavanone synthase. These data were used to calculate the expected, light-induced changes in enzyme activity from the measured changes in mRNA activity. The results for both enzymes were in agreement with experimental data, indicating that the light-induced changes in enzyme activity can be explained by changes in the respective mRNA activity. Some data concerning changes in the degree of polyadenylation of the mRNAs and the inhibitory effects of 7-methylguanosine 5'-phosphate are presented.