Choline/ethanolamine kinase (ChoK/EtnK) exists as at least three isoforms (α1, α2 and β) in mammalian cells. The physiological significance for the existence of more than one form of the enzyme, however, remains to be determined. In the present study, we examined the expression and distribution of the isoforms in mouse tissues using isoform-specific cDNA probes and polyclonal antibodies raised against each N-terminal peptide sequence. Both Northern- and Western-blot analyses indicated that either the α (α1 plus α2) or the β isoform appeared to be the ubiquitously expressed enzyme. The mRNA abundance for the α isoform was highest in testis, whereas that for the β isoform was relatively high in heart and liver. While the native form of each isoform was reported to consist of either homodimers or homotetramers, our immunotitration studies clearly indicated that a considerable part of the active form of the enzyme consists of α/β hetero-oligomers, with relatively small parts of activity expressed by α/α and β/β homo-oligomers. This is the first experimental evidence for the presence of heteromeric ChoK/EtnK in any source. Thus our results strongly suggested that the activity of ChoK/EtnK in the cell is controlled not only by the level of each isoform but also by their combination to form the active oligomer complex. Carbon tetrachloride (CCl4) was shown to induce ChoK activity 2–4-fold in murine liver. Our analysis for the mechanism involved in this induction revealed that the responsible isoform for CCl4 was α, not β. The level of α mRNA was strongly induced in mouse liver, which resulted in a sustained increase in the amount of the α isoform. Consequently, the composition of α/α homo-oligomers came to represent up to 80% of the total active molecular form of ChoK in CCl4-induced liver, whereas it was less than 20% in normal uninduced liver.