Cells of Mucor racemosus were labeled with l-[(14)C]leucine during the yeast-to-hyphae morphogenesis that follows a change of atmosphere from CO(2) to air. Pulse-labeling kinetics and the steady-state accumulation of incorporated l-[(14)C]leucine were determined throughout the period of cellular differentiation. We determined that the l-[(14)C]leucine was taken up by all forms of the organism, was not altered from the form of l-leucine, and was incorporated exclusively into protein. The intracellular pool of free l-leucine was small in comparison with those of the other l-amino acids, remained relatively constant in size during morphogenesis, and was rapidly equilibrated with exogenous leucine. Approximately the same internal radiospecific activities were attained throughout development shortly after addition of l-[(14)C]leucine to a culture. Experiments performed with leucine auxotrophs suggested that endogenous synthesis of leucine in prototrophs does not affect the measured rates of incorporation. Experiments performed with (14)C-labeled l-isoleucine, l-proline, l-lysine, and l-arginine produced results qualitatively the same as with l-leucine. The accumulation of incorporated l-[(14)C]leucine in a culture of M. racemosus undergoing the air-induced yeast-to-hyphae transition reflected the change in growth rate that accompanied the morphogenesis. However, the specific rate of protein synthesis measured throughout the developmental process displayed a characteristic acceleration during the emergence of germ tubes which was followed by a decline when all further growth took the form of hyphal elongation. Data are presented suggesting that this response is a correlate of morphogenesis rather than a consequence of the atmospheric change per se.