Phospholipid turnover increases with both mitogenic stimulation and oncogenic transformation (1-9). Recent 31P nuclear magnetic resonance (NMR) spectroscopy studies of human tumors, animal tumor models and cell systems have reported elevated phosphomonoesters with growth and oncogenic transformation, as well as changes in these levels associated with treatment (10). In order to gain insights into the mechanisms underlying these changes, we used a phosphonium analog of choline and 31P NMR spectroscopy to study choline metabolism in quiescent and mitogenically stimulated Rat-2 fibroblasts. Cell growth status of these cells has a significant effect on choline metabolism. While overall uptake of the analog was similar in both quiescent and growing cells, distribution among metabolite pools differed. Quiescent cells accumulate label in the phosphodiester pool, with little or none in the phosphomonoester pool. On the other hand, mitogenic stimulation resulted in a significant fraction of the label in the phosphomonoester pool.