1. N1E-115 mouse neuroblastoma cells morphologically differentiate by extending neurites in a period of seven days after addition of 2% DMSO to the culture medium. We used the whole-cell patch clamp technique to measure calcium currents in these cells under conditions where voltage clamp of the whole membrane was assured. 2. Current densities of both T and L type calcium currents were identical in cells included to differentiate with dibutyryl cyclic AMP and cells induced to differentiate with dimethylsulphoxide (DMSO). Cells differentiated with DMSO were used for all subsequent experiments. 3. All morphologically differentiated cells showed a T type calcium current. In contrast, a minority of morphologically undifferentiated cells did not show a T current. 4. Once expressed, both T and L currents did not change either in current density or in behaviour over a period of five days. 5. These data demonstrate that expression of a T current always precedes neurite extension, and suggest a role for calcium currents in triggering morphological differentiation.