Basic peptides such as human immunodeficiency virus type 1 (HIV-1) Tat-(48-60) and Drosophila Antennapedia-(43-58) have been reported to have a membrane permeability and a carrier function for intracellular protein delivery. Based on the fluorescence microscopic observations of the vascular endothelial cells (ECV-304) and the primary cultured neuroglial cells, we found that human Clock protein DNA-binding peptide [residue 35-47, hClock-(35-47)] had a translocation activity very similar to Tat-(48-60). The cellular uptake of hClock-(35-47) increases with the increase of incubation time and concentration. The internalization effect at 4 degrees was same as that at 37 degrees C. Internalization of hClock-(35-47) was saturable and could be inhibited by the excess of the other MPPs. Moreover, the uptake of these peptides were significantly inhibited in the presence of heparan sulfate. These results strongly suggested that the hClock-(35-47) shared a common or very similar internalization pathway with other MPPs. Furthermore, we injected rat through the common carotid artery with hClock-(35-47)-FITC peptide, and cryostat sections of the brain were prepared and observed using a fluorescence microscope. Result showed that the peptide had the ability to translocate through the blood-brain barrier. It is promising to provide a new safe carrier for the intracellular and encephalic treatment.