1. The facilitatory effect of external calcium ions (Ca2+o) on the alpha1-adrenoceptor-activated non-selective cation current (Icat) was investigated in rabbit portal vein cells using noise and voltage-jump relaxation analysis of the whole-cell macroscopic current. 2. Micromolar concentrations of Ca2+o potentiated the peak amplitude of Icat at a holding potential (Vh) of -50 mV. The effective [Ca2+]o which produced a 50% potentiation (EC50) was 3 microM. 3. From noise analysis the estimated single channel conductance (gamma) was approximately 23 pS with [Ca2+]o between 3 and 100 microM, whereas in < 10 nM or 1 microM Ca2+o gamma was approximately 10 pS. 4. The spectral density function of Icat at negative potentials could be described by the sum of two Lorentzians in every [Ca2+]o examined. The time constant of the lower frequency Lorentzian component (tau1) was about 11 ms in < 10 nM Ca2+o and was about 45 ms in micromolar concentrations of Ca2+o (1-100 microM). In contrast, the time constant of the higher frequency component (tau2) was similar in < 10 nM Ca2+o and 100 microM Ca2+o (between 1 and 2 ms). 5. The lower frequency Lorentzian component was responsible for about half the total current variance in < 10 nM Ca2+o whereas in micromolar concentrations of Ca2+o it was responsible for most of the measured current variance. 6. In voltage-jump experiments, on stepping the voltage from -50 to +50 mV the instantaneous current was followed by an exponential decline of Icat. Stepping back to -30 mV produced an exponential inward relaxation (Irelax,-30 mV) leading to an increase in the steady-state amplitude of Icat in micromolar concentrations of Ca2+o, but this relaxation was not observed in < 10 nM Ca2+o. The relative amplitude of Irelax,-30 mV increased in an [Ca2+]o-dependent manner (EC50 was 2 microM) although the time constant of this relaxation (taurelax,-30 mV) remained unchanged (about 60 ms between 2 and 100 microM Ca2+o). 7. The data suggest that Ca2+o produces marked changes in the kinetics and single channel conductance of cation channels, which may account for the facilitatory effect of micromolar concentrations of Ca2+o on the peak amplitude of Icat.