Cotton ovules cultured in an insufficiency of boron (10 micromolar), showed inhibition of fiber growth by the ninth day in culture. Averaging data from eight to eleven days of culture under these conditions, total incorporation of [6-(14)C]orotic acid into fiber was inhibited by 59%. Inhibition was evident in all radioactively labeled pools, indicating that the effect may be at the membrane transport level or at an early stage of orotic acid metabolism. On a per cent basis, incorporation into RNA under boron deficiency was higher than under sufficiency. The effect is greater on the eighth day of culture, with a decreasing difference from controls up to the eleventh day. Conversely, the per cent incorporation into UDP-glucose was lower under boron deficiency than in controls, having a more or less constant value from 8 to 11 days of culture. Thus, a primary event of boron deficiency in cotton fiber culture is an alteration in the flow of metabolites through the pyrimidine synthesis pathway.