Protein synthesis was studied in the thoracic ganglia of the cockroach. All the motor neurons leaving the metathoracic ganglion were axotomized by cutting the nerve roots. In vivo experiments demonstrated that within two days of the operation those ganglia with cut nerve roots had an increased [3H]leucine incorporation into protein resulting from an increased uptake and labeling of the leucine pool. These effects were non-specific responses to the damage and trauma produced by the operation. In vitro labeling experiments failed to demonstrate any change in incorporation or uptake of [3H]leucine up to 16 days after the operation. Autoradiographic experiments were done on ganglia in which only nerve root 5 on one side was cut. Grain densities and cytoplasmic areas were measured for the ipsilateral, axotomized, identified, fast motor neuron, Df and the homologous, contralateral, intact cell of the same ganglion. The first statistically significant increase in grain density in the axotomized Df was observed 12 days after the operation. This may be compared with the 13 day lag period after the operation before the nerves start growing from the proximal stump. The significance of this relatively long lag period before the increased [3H]leucine incorporation and the start of nerve growth is discussed in terms of the ability of regenerating neurons to specifically reinnervate the cells to which they were originally connected.