To investigate the control of the timing in the epithelio-mesenchymal transformation of the neural crest into a migrating population, neural anlagen (neural tube plus crest) were isolated from 2-day quail embryos by proteases in the presence of CA++ and explanted onto substrates favourable for neural crest cell migration. Explants isolated before normal migration had commenced required 3-8 h in vitro before neural crest cells started migration, but explants obtained at migratory stages showed an immediate onset of migration. The schedule was similar to that expected in vivo. When pre-migratory neural anlagen were isolated by protease in Ca++-and Mg++-free (CMF) medium, or when the protease was followed by a brief (5 min) exposure to CMF medium, neural crest cell migration commenced without delay, and the cohesion of the anlagen was impaired. Ca++-free medium duplicated the effects of CMF, but neither Mg++-free medium nor CMF treatment before treatment with protease stimulated migration and reduced cohesion. Precocious neural crest cell migration and reduced cohesion also followed when neural anlagen of pre-migratory stages were cultured with membrane Ca++-channel antagonists D600 and Nifedipine, without any exernal Ca++-depletion. The decrease of cohesion of these tissues is consistent with results in other systems where protease/Ca++-depletion inactivates Ca++-dependent cell-cell adhesive mechanisms. Therefore, we suggest that Ca++-dependent cell-cell adhesions play a part in preventing neural crest cells from migrating precociously and that the timed inactivation of this adhesion system normally helps trigger the onset of migration. The results with blockers of Ca++-channels suggest that Ca++ levels may be involved in regulating this system.