Filopodia have been regarded as the sensory extensions of neuronal growth cones. As such, filopodia assay distant environments and are important for directing growth cones toward their targets. Since the territory encountered by a growth cone depends on the area spanned by the filopodia, changes in filopodial length or number result in the “exploration” of different-sized regions of the environment. The present study tests the potential regulatory role of intracellular calcium levels ([Ca2+]i) on filopodial morphology in identified neurons from the snail Helisoma. Experimentally evoked changes in [Ca2+]i were measured with the fluorescent calcium indicator fura-2 and directly correlated with growth cone filopodial morphology. A rise in [Ca2+]i caused two distinct, concentration-dependent effects separable by their different time courses: within the first 10 min, filopodia underwent significant elongation, while the second phase was characterized by a massive loss of filopodia. Both of these behaviors were increased in a calcium-dependent fashion. The magnitude of both filopodial elongation and filopodial loss correlated well with the transient peak values of [Ca2+]i reached during a given experimental treatment (r less than or equal to 0.98). In addition to the direct effect of the initial transient rise in [Ca2+]i, there is evidence for a form of adaptation of filopodial behavior to sustained calcium levels. A transient change in [Ca2+]i of as little as 30-50 nM reliably altered filopodial morphology. These results indicate that even small changes in intrinsic calcium homeostatic properties or extrinsic signals that alter intracellular calcium levels can act as regulators of the size of the environment sampled by an elongating growth cone.