The induction of Per1 gene in the suprachiasmatic nucleus, the center of the circadian clock, is assumed to play significant roles in the adjustment of the internal clock. cAMP is one of the intracellular mediators which activates Per1 transcription. Here, we showed that the amount of the rat Per1 (rPer1) transcript induced by forskolin (FK) was significantly upregulated by the inhibition of phosphodiesterase type 4 (PDE4), a specific phosphodiesterase for cAMP, in rat-1 fibroblasts. Administration of rolipram, a specific inhibitor of PDE4, increased intracellular cAMP concentration, phosphorylation of cAMP response element binding protein (CREB) and enhanced rPer1 induction at their peaks. However, in the falling phase of rPer1 induction, the inhibition of PDE4 hardly affected the profile of rPerl expression. These findings suggest the involvement of PDE4 for the regulation of rPer1 expression via cAMP metabolism at peak of the induction but little or no participation of PDE4 in the decreasing phase of the gene expression. (C) 2003 Elsevier Science (USA). All rights reserved.