In order to assess the effects of [K+]o on the passive membrane properties of neostriatal neurons, the cable properties of these cells were determined at two extracellular potassium concentrations (6.25 and 3.0 mM). The effect of tetraethylammonium (TEA) on cable properties was also studied at 6.25 [K+]o. At 6.25 mM [K+]o, the mean input resistance at the resting membrane potential (RMP), and the mean membrane time constant (tau o) were 27 +/- 1.5 M omega and 6.9 +/- 0.5 ms respectively (n = 17), while at 3 mM [K+]o they were 62.9 +/- 4.8 M omega and 14.3 +/- 0.6 ms (n = 15) (mean +/- SEM). With one of the methods used to calculate the electronic parameters, the total electrotonic length of the dendrites (L) and the dendritic to somatic conductance ratio (rho) were 1.3 +/- 0.05 and 5 +/- 0.8 at the higher [K+]o respectively, while they were 0.95 +/- 0.04 and 3 +/- 0.7 at the lower [K+]o. Cells were depolarized in 6.25 as compared to 3 mM [K+]o (RMP = -66 +/- 1.3 mV vs RMP = -80.5 +/- 1.4 mV). After one hour exposure to TEA (10 mM), the input resistance and time constant tripled at 6.25 mM [K+]o. TEA slightly depolarized the cells bathed in 6.25 mM [K+]o. The results suggest that changes in [K+]o, within the physiological range, markedly affect the cable properties of neostriatal neurons, possibly modifying subthreshold, voltage-dependent K+-conductances. TEA seems to block some of these channels.