By using voltage clamping and microiontophoretic techniques, it has been found that the prolonged cholinergic and dopaminergic inhibition seen in Aplysia burst firing neurons occludes the inward current on which slow oscillations depend. It also mimics the temperature and ionic sensitivity of that inward current. This prolonged inhibition, which cannot be inverted and is insensitive to extracellular potassium changes, thus appears to result from a synaptically produced inactivation of the regenerative slow inward current underlying bursting.