The neurotubule protein content of chick embryo 8-day dorsal root and 14-day sympathetic ganglia, induced to extend neurites in the presence of Nerve Growth Factor, was determined by the time-decay colchicine binding assay procedure and by two independent polyacrylamide gel electrophoresis systems. The initial level of neurotubule protein in dorsal root ganglia was approximately 16% of the total soluble protein. This value was constant during Nerve Growth Factor-mediated neurite outgrowth. The initial level of neurotubule protein in sympathetic ganglia was also approximately 16%, and was unchanged during neurite outgrowth. In addition, C1300 mouse neuroblastoma cells, induced to extend neurites in 0.1% serum, also did not exhibit a change in neurotubule protein concentration, which remained approximately 9% of the total soluble protein.