The electroresponsive properties of guinea-pig thalamic neurones were studied using an in vitro slice preparation. A total of 650 cells were recorded intracellularly comprising all regions of the thalamus; of these 229 fulfilled our criterion for recording stability and were used as the data base for this report. The resting membrane potential for thirty-four representative neurones which were analysed in detail was -64 +/- 5 mV (mean +/- S.D.), input resistance 42 +/- 18 M omega, and action potential amplitude 80 +/- 7 mV. Intracellular staining with horseradish peroxidase and Lucifer Yellow revealed that the recorded cells had different morphology. In some their axonal trajectory characterized them as thalamo-cortical relay cells. Two main types of neuronal firing were observed. From a membrane potential negative to -60 mV, anti- or orthodromic and direct activation generated a single burst of spikes, consisting of a low-threshold spike (l.t.s.) of low amplitude and a set of fast superimposed spikes. Tonic repetitive firing was observed if the neurones were activated from a more positive membrane potential; this was a constant finding in all but two of the cells which fulfilled the stability criteria. The l.t.s. response was totally inactivated at membrane potentials positive to -55 mV. As the membrane was hyperpolarized from this level the amplitude of the l.t.s. increased and became fully developed at potentials negative to -70 mV. This increase is due to a de-inactivation of the ionic conductance generating this response. After activation the l.t.s. showed refractoriness for approximately 170 ms. Deinactivation of l.t.s. is a voltage- and time-dependent process; full de-inactivation after a step hyperpolarization to maximal l.t.s. amplitude (-75 to -80 mV) requires 150-180 ms. Membrane depolarization positive to -55 mV generated sudden sustained depolarizing 'plateau potentials', capable of supporting repetitive firing (each action potential being followed by a marked after-hyperpolarization, a.h.p.). The a.h.p. and the plateau potential controlled the voltage trajectory during the interspike interval and, with the fast spike, constitute a functional state where the thalamic neurone displayed oscillatory properties. Frequency-current (f-I) plots from different initial levels of membrane potential were obtained by the application of square current pulses of long duration (2s). From resting membrane potential and from hyperpolarized levels a rather stereotyped onset firing rate was observed due to the presence of the l.t.s.(ABSTRACT TRUNCATED AT 400 WORDS)