Quasi steady-state electric fields were applied across the isolated turtle cerebellum to study the relationship between applied field, neuronal morphology and the modulation of the neuronal spike firing pattern. Spiking elements were identified electrophysiologically using extracellular recording methods and by subsequent horseradish peroxidase injection, which revealed their dendritic morphology and orientation. The electric field was precisely defined by measuring the voltage gradients induced in the cerebellum by 40 s constant-current pulses. The field was constant in the vertical (dorso-ventral) axis and zero in the horizontal plane, in agreement with theory. Neurones were modulated by applying a sinusoidal field at frequencies between 0.05 and 1.0 Hz. Modulated cells exhibited an increase in firing frequency and fell into one of four classes, depending on the direction of the field that produced the modulation. Thus neurones were excited by: ventricle-directed fields (V modulation), pia-directed fields (P modulation), both of the above (V/P modulation) or showed no consistent modulation (non-modulation). Most Purkinje somata and primary dendrites (nineteen out of twenty-eight) and most Purkinje dendrites (eighteen out of thirty), were V modulated with maximum rate proportional to the peak field intensity. The dendrites of these cells were consistently oriented toward the pia. Among the stellate cells, the lower molecular layer stellates, with dendrites extending predominantly towards the pia, were mostly (nineteen out of thirty-two) V modulated. The mid-molecular layer stellates, which showed much variability in dendritic orientation, were distributed among all four of the modulation classes. The upper molecular layer stellates, with a mostly horizontal dendritic alignment, were mainly (nine out of sixteen) non-modulated. All groups of spiking elements showed a correlation between patterns of modulation by applied fields and dendritic orientation, which suggests the degree of differential polarization of the extended cable elements of the neurone by the applied field as the basic mechanism for field-induced excitation or inhibition. The threshold for modulation among all neurones was 15-20 mV/mm, which is similar to the fields that modulate other nervous tissues. This suggests that many neurones can be modulated by fields of the order of 10-20 mV/mm.