Circadian activity rhythms in mammals are controlled by the expression and transcriptional regulation of clock genes in the suprachiasmatic nucleus (SCN). The circadian cycle length in hamsters is regulated in part by casein kinase I epsilon (CKIε). A semidominant mutation (C→T, R178C, CKIεtau) appears to act as a dominant-negative allele to shorten the period of circadian rhythms. We tested this hypothesis in vivo by expressing wild-type CKIε gene in homozygous tau mutant hamsters. High-level CKIε+/+ gene transfer and expression (as indicated by green fluorescent protein) were obtained by injecting CKIε-containing plasmids bilaterally near the SCN, followed by in vivo electroporation. Rhythmicity reappeared 5–7 days after electroporation, with a gradual increase in circadian period over the next 10 days. The circadian period returned to the baseline over the next 20 days. For the five hamsters with clearest gene expression in the SCN, the mean lengthening time was 39.6 min. Period change was not observed in either control tau mutant hamsters electroporated with plasmids lacking the CKIε gene or in wild-type hamsters with plasmids containing the wild-type CKIε gene. Therefore, normal periodicity in homozygous CKIεtau hamsters was partially rescued by expression of the wild-type CKIε gene in the SCN, supporting a competitive and dominant-negative action of the mutant allele. This study shows that electroporation of wild-type CKIε gene into the SCN is sufficient for lengthening the shorter circadian period of tau mutant hamsters in a time-dependent way and supports the conclusion that CKIεtau is the cause of the shorter period.