Measurements of the lactose repressor over a tenfold range of cell growth rates were made on protein extracts from Escherichia coli cultures grown in media with various carbon energy sources. The concentration of lactose repressor varied with the number of genome equivalents per cell over this range in growth rates, suggesting that the number of lactose molecules within the cell is determined by the number of I gene copies present. The timing of repressor synthesis during the cell division cycle and its correlation with deoxyribonucleic acid synthesis was examined by synchronizing the cell division cycle of E. coli ED1039, in which the Lac region has been transposed from 10 to 36 min on the genetic map. Measurements of lactose repressor in the synchronized culture revealed a burst of repressor synthesis at the time of I gene duplication. The concentration of lactose repressor was found to decrease as a function of total cell protein during the division cycle until an increase in synthesis occurred, suggesting that repressor synthesis probably does not occur throughout the division cycle. A model for I gene regulation is proposed.