Background: [Methyl-H-3]-choline is a promising new positron emission tomography (PET) agent used for cancer imaging whose mechanism has still not fully been elucidated. In this study, whether [methyl-H-3]-choline determined by measuring the activity of choline kinase (ChoK) and phospholipase D (PLD) in rapidly proliferating and confluent breast cancer MCF-7 cells is related to cell proliferation or not was investigated. Materials and Methods: The activity of ChoK and PLD were determined using ion exchange chromatography and transphosphatidylation assay respectively. Results: [Methyl-H-3]-PCho content expressed as pmol mg(-1) protein(-1) min(-1) (n=6) was significantly higher in the exponentially growing (484.04 +/- 20.23) compared with confluent (70.35 +/- 9.83) cells using Student’s t-test (p < 0.001). Moreover, PLD activity expressed as the mean (n=6) (disintegration per minute (d.p.m.)/mu g protein +/- SD (mean S phase SD)) showed significantly higher (p < 0.001) activity in the exponentially growing cells (196.39 +/- 2.21 d.p.m./mu g protein (39.69 +/- 4. 00%)) compared with confluent cells (99.10 +/- 1.35 d.p.m./mu g protein (9.33 +/- 0.82%)). Conclusion: This study indicates that the major water-soluble choline metabolite was phosphocholine (PCho) as a consequence of increased ChoK and PLD activity in the exponentially growing cells compared to confluent cells.