1. To characterize the physiological properties of lateral and basolateral (BL) amygdaloid neurons, intracellular recordings were performed in barbiturate-anesthetized cats. Morphological identification of recorded cells was achieved by intracellular injection of neurobiotin. Two types of physiologically identified projection neurons were distinguished in the BL and lateral nuclei. 2. The first type of neurons prevailed in the BL nucleus (80% of BL cells). Their resting membrane potential (Vm) averaged -66 +/- 4.9 (SE) mV. They generated stereotyped spike doublets or bursts in response to threshold depolarizing pulses. In most cells, depolarizing pulses of higher amplitude elicited spike bursts or doublets at a shorter latency followed by a nonadapting train of single spikes whose frequency rose with the amplitude of the current pulses. However, 15% of BL bursting neurons generated repetitive spike bursts or doublets in response to prolonged depolarizing current pulses. The response of BL bursting neurons to hyperpolarizing current pulses revealed the presence of slow inward rectification in the form of a depolarizing sag, thus suggesting the presence of a hyperpolarization-activated current. 3. The second type of neurons prevailed in the lateral nucleus. Their resting Vm was quite polarized (-74 +/- 2.85 mV) and they generated slow Vm oscillations (2-10 Hz) upon steady depolarization beyond congruent to -62 mV. The frequency of the oscillation increased with the amount of depolarizing current. In the majority of cells, analysis of voltage responses to subthreshold current pulses revealed the presence of voltage- and time-dependent rectification in the depolarizing direction. Current pulses that brought the Vm to -65 mV and beyond elicited a voltage response that reached an early peak and then decayed. Increasing the amplitude of the pulse decreased the latency of the early peak until it triggered an action potential. Current-voltage plots demonstrated inward rectification in the depolarizing direction. At the break of hyperpolarizing current pulses applied at depolarized levels, the Vm overshot prepulse values and generated one or more oscillatory cycles. 4. An important proportion of bursting and oscillating neurons (45.8% and 29%, respectively) were physiologically identified as projection neurons by antidromic invasion from the basal forebrain, entorhinal cortex, or perirhinal cortex. The conduction velocity of bursting and oscillating neurons estimated from the latency of antidromic spikes was low (< or = 2.5 m/s). 5. Most bursting and oscillating neurons of the BL nucleus were spiny cells with a pyramidal morphology. Four to eight dendritic trunks emerged from the apex, base, and sides of their triangular soma.(ABSTRACT TRUNCATED AT 400 WORDS)