The level of mRNA for S-adenosylmethionine decarboxylase (SAMDC) (EC 4.1.1.50) in leaves of Pharbitis nil exposed to light began to increase after 15 min exposure, peaked by 45 min and was followed by a rapid decline, suggesting a very rapid light-regulation of the gene expression. These changes of the mRNA level indicate that the photoresponse of SAMDC abundance is regulated primarily at a transcriptional level.