An increase in intracellular free calcium concentration ([Ca2+]i) was observed following the application of nicotine to isolated adult dorsal unpaired median (DUM) neurons of the cockroach (Periplaneta americana) terminal abdominal ganglion (TAG) using Fura-2 fluorescence measurements. Bath-applied nicotine (1 mM) induced a transient increase in [Ca2+]i. Calcium responses to bath-applied nicotine were blocked completely by alpha-bungarotoxin (100 nM) and were reduced by 50% in the presence of pirenzepine (1 microM). The sensitivity of the response to both nicotinic and muscarinic antagonists suggested that it was mediated by an acetylcholine receptor with 'mixed' pharmacology. In whole cell current-clamp experiments, nicotine reduced the frequency of evoked action potentials by decreasing the slope of the predepolarization in the last two-thirds of the pacemaker potential. Voltage-clamp studies revealed that nicotine modified the inactivation properties of the maintained low-voltage-activated (LVA) calcium current increasing the rate of relaxation of this current and transforming a U-shaped voltage dependence of inactivation into a monotonic relationship to voltage. These effects were blocked when isolated DUM neurons were pretreated with 0.5 microM alpha-bungarotoxin. Our findings suggested a novel calcium-dependent regulation of firing behavior in TAG DUM neurons following activation of an acetylcholine receptor with 'mixed' pharmacology, resulting in a rise in [Ca2+]i which reduces firing frequency by modulating a maintained LVA calcium current responsible for the action potential predepolarization.