Butyrylcholinesterase was found in chick sciatic nerve in four main molecular forms--G1, G2, G4 and A12--distinguishable by thier sedimentation coefficients in sucrose gradients (4.2S, 6.4S, 11.3S and 19S, respectively). Axonal transport of butyrylcholinesterase was studied by measuring the accumulation of its molecular forms on each side of a transected sciatic nerve. Twenty-four hours after transection, butyrylcholinesterase activity had risen by about 32% at the extremity of the proximal stump, and by 20% at the extremity of the distal stump. Proximal accumulation was due to a two-fold rise in G4 activity and to a six-fold rise in A12 activity, whereas distal accumulation was exclusively due to a 50% increase in G4 activity, accompanied by the complete loss of A12. The activities of G1 and G2 remained stable in both directions. Under our experimental conditions, the accumulation of butyrylcholinesterase activity cannot be attributable to local protein synthesis, cross-contamination with accumulated acetylcholinesterase or the presence of plasma butyrylcholinesterase. Hence we conclude that all A12 butyrylcholinesterase molecules were carried in the anterograde direction, moving at 11.6 +/- 4.2 mm/day, and that probably some of the G4 molecules were slowly transported in both directions. These findings suggest that some of the butyrylcholinesterase is located in the axonal mitochondria and/or axolemma.