Assembly of chick brain tubulin onto flagellar microtubules from Chlamydomonas and sea urchin sperm. Flagellar microtubules from Chlamydomonas and sea urchin sperm were used as in vitro assembly sites for chick brain tubulin. Brain microtubules assembled onto the A-tubules, central tubules, and, to a limited extent, onto the distal ends of the axonemes at low tubulin concentrations and onto distal and proximal ends at high tubulin concentrations; however, the rate of assembly onto the distal end was always greater. The rate of neurotubule assembly onto axonemes was shown to be dependent upon tubulin concentration and a forward rate constant for assembly was determined.