DNA, RNA and protein synthesis were studied by autoradiography in cultured keratinocytes, immediately, 24 and 48 h after 8-methoxypsoralen and ulraviolet light, (PUVA) treatment. Using the same technique, the immediate and long-term effects of PUVA therapy on DNA, RNA and protein synthesis were analysed in skin biopsies from psoriatic patients. In the cultures, immediately after irradiation, DNA and RNA syntheses were similarly inhibited in a dose-dependent manner, while protein synthesis was slightly affected only for the highest dose. After 24 h, RNA synthesis recovered whereas DNA synthesis was more severely inhibited suggesting that other cell components may be damaged by PUVA. In patients, DNA and RNA syntheses decreased immediately after PUVA sessions. During all the sessions until the psoriatic plaques had cleared, an impairment of DNA synthesis was observed in comparison with the synthesis in involved and uninvolved skin before treatment. These results suggest that the therapeutic efficiency of PUVA is based on the inhibition of DNA replication due to direct effects on nucleic acids but also to photoreactions with other cell components.