During development and regeneration of the nervous system, growth cones of the various nerve cells navigate and direct neurite elongation by detecting and responding to cues in the environment. To investigate changes in growth cone behaviour due to calcium influx we used nerve growth factor (NGF)-induced growth cones of PC12 (rat pheochromocytoma cells) cells as a model. High external concentrations of potassium and ATP depress growth cone motility, induce club-shaped growth cones and reduce filopodia length and the number and relative F-actin contents of single growth cones (r.a.c.), respectively. The cellular responses are mediated by a sustained increase in the intracellular free Ca2+ concentrations ([Ca2+]i) as monitored by calcium-sensitive fluorescent dyes and confocal microfluorimetry. The responses are not detectable in the presence of the protein tyrosine kinase inhibitor genistein. Immunocytochemistry revealed an increased level of tyrosine-phosphorylated proteins in cell bodies and growth cones but not in cell nuclei. Paxillin, a cytoskeleton-associated protein located in neurites and growth cones, was detected among the phosphotyrosine proteins. The sustained (> 30 s) Ca2+ influx through voltage-gated L-type but not N- or P-type Ca2+ channels induced the F-actin loss and tyrosine phosphorylation. Ca2+ entry through P2X2 ligand-gated channels caused the same effects. Our data suggest the following mechanism: increased [Ca2+]i levels activate tyrosine kinases located close to the ion channels which then leads to changes in morphology due to tyrosine phosphorylation of proteins, e. g. paxillin.