Whole-cell voltage clamp techniques were used to characterize sustained outward currents in maturing (P4 to P48) acutely isolated rat CA1 hippocampal neurones. Sodium removal and signal subtraction were used to isolate a sodium dependent sustained potassium current (IKNa). Calcium blockade (Co2+), sensitivity to a low TEA dose (0.5 mM) and sensitivity to Charibdotoxin (CTX 25 nM) and Iberiotoxin (IbTX 25 nM), in conjunction with signal subtraction, were used to isolate a sustained current with the characteristics of IC (IKCa). IKNa was found in both immature (P4-5) and older (P > 21) cells; this corresponded, respectively, to 56 +/- 5% and 36 +/- 6% of the outward current in younger and older cells. In the course of maturation, the voltage dependence of activation of IKNa shifted to more hyperpolarized values by approximately 20 mV. In the younger cells (P5-18) there was no evidence for sensitivity to CTX or IbTX. In 55 out of 77 older cells we found a component sensitive to CTX, IbTX, 0.5 mM TEA and Co2+.