The Retzius neuron from the leech, growing in culture on the plant lectin concanavalin A as substrate, produces broad flat growth cones and thick bundles of processes. The same cell extends fine straight processes with numerous branches when grown on a laminin-like substrate extracted from leech central nervous system extracellular matrix, referred to as "leech laminin extract." Cells growing on these two different substrates also show marked differences in the pattern of Ca2+ entry following evoked impulses, as detected optically by local changes in absorbance of the Ca2+-sensitive dye arsenazo III. Ca2+ enters the soma and initial segment of Retzius cells grown on both substrates. However, detectable Ca2+ entry only occurs into the processes of cells growing on leech laminin but not of those growing on concanavalin A. Optical recordings of changes in membrane potential made with the voltage-sensitive dye RH 155 taken from cells growing on either substrate indicate that a depolarization initiated in the soma spreads to the most distant processes with little or no distortion in amplitude or time course. This implies that all voltage-sensitive Ca2+ channels in the cell membrane are equally activated by depolarizing stimuli. Therefore, the fact that impulses evoke Ca2+ entry into processes of Retzius cells grown on leech laminin extract but not of cells grown on concanavalin A shows that the nature of the growth substrate can affect the number and distribution of their functional Ca2+ channels.