Marked changes in the activity of the 'de novo' and 'salvage' pathways of pyrimidine biosynthesis during growth of Vinca rosea cells in a batch suspension culture were observed.The activity of these pathways was investigated by determining the contribution of I4C of [2-MC|uracil, [2-MC]uridine. and [6-14C]orotate to the cell constituents and by measuring the activity of the several enzymes of these pathways.During the lag phase of the culture, 'uracil-' and 'uridine-salvage' pathways made the predominant contribution to nucleotide biosynthesis, but, following the initiation of cell division, the 'de novo' pathway for nucleotide biosynthesis operated appreciably.These results suggest that nucleotide synthesis during cell growth in a suspension culture can be divided into two stages: a 'turnover stage', during the lag phase of cell growth, and a 'true biosynthetic stage', which is initiated in the cell division phase.