Potassium currents in cells isolated from myometrium of immature (23-26 days after birth) rats were measured by whole-cell patch clamp to analyse the effects of gonadal steroids. Predominantly outward K+ currents with an early transient component were recorded in response to depolarizing pulses from a holding potential of -90 mV. The transient current, which was rarely present in adult myometrial cells, was inactivated by holding the membrane potential at -40 mV and attenuated by 1 mM 4-aminopyridine. Under these conditions a second sustained outward current was apparent. Administration of 17 beta-oestradiol to the immature rat before isolation of the myometrial cells reduced the probability of occurrence of the transient currents from 78.6% of cells (n = 33) to 29.8% (n = 14). Progesterone had only a slight effect. The kinetics of the transient currents were differently influenced by the two steroids. Cells isolated from animals previously given 17 beta-oestradiol showed transient outward currents with significantly shorter time constants of decay than those for control cells. Conversely, progesterone caused the time constants to be increased. We conclude that the gonadal steroids exert an influence on the expression of different populations of ionic channels in isolated cells of the immature rat uterus. These channels may regulate the excitability and contractility of the uterus in vivo.