Circadian rhythms are endogenous cycles with periods close to, but not exactly equal to, 24 h. In mammals, circadian rhythms are generated in the suprachiasmatic nucleus (SCN) of the hypothalamus as well as several peripheral cell types, such as fibroblasts. Protein kinases are key regulators of the circadian molecular machinery. We investigated the role of the c-Jun N-terminal kinases (JNK), which belong to the mitogen-activated protein kinases family, in the regulation of circadian rhythms. In rat-1 fibroblasts, the p46 kDa, but not the p54 kDa, isoforms of JNK expressed circadian rhythms in phosphorylation. The JNK-inhibitor SP600125 dose-dependently extended the period of Period1-luciferase rhythms in rat-1 fibroblasts from 24.23+/-0.17-31.48+/-0.07 h. This treatment also dose-dependently delayed the onset of the bioluminescence rhythms. The effects of SP600125 on explant cultures from Period1-luciferase transgenic mice and Period2(Luciferase) knockin mice appeared tissue-specific. SP600125 lengthened the period in SCN, pineal gland, and lung explants in Period1-luciferase and Period2(Luciferase) mice. However, in the kidneys circadian rhythms were abolished in Period1-luciferase, while circadian rhythms were not affected by SP600125 treatment in Period2(Luciferase) mice. Valproic acid, already known to affect period length, enhanced JNK phosphorylation and, as predicted, shortened the period of the Period1-bioluminescence rhythms in rat-1 fibroblasts. In conclusion, our results showed that SP600125 treatment, as well as valproic acid, alters JNK phosphorylation levels, and modulates the period length in various tissues. We conclude that JNK phosphorylation levels may help to set the period length of mammalian circadian rhythms.