1. An inwardly rectifying potassium current expressed in Xenopus laevis oocytes injected with canine hippocampal poly(A)+ RNA was investigated with the two-microelectrode voltage clamp technique. 2. Xenopus oocytes injected with canine hippocampal poly(A)+ RNA expressed a current activated by hyperpolarization. This current contained an instantaneous and a time-dependent component. Both components were inwardly rectifying and could be blocked by extracellular Cs+ or Ba2+. 3. The expressed current was carried mainly by K+. Its reversal potential measured in different [K+]os could be fitted by the Nernst equation with a slope of -50.7 per tenfold change in [K+]o. Extracellular Cl- and Na+ made minimal contributions to the current. 4. The activation of the expressed current depended on both voltage and [K+]o. Activation started near EK and the activation curve shifted along the voltage axis in parallel with EK when [K+]o was altered. 5. The activation time constants of the expressed current also depended on both voltage and [K+]o. The voltage dependence of the time constants was bell-shaped and the peak value was at a potential 30-50 mV more negative than EK. The voltage dependence of the time constants shifted along the voltage axis when EK was changed. 6. The poly(A)+ RNA extracted from canine hippocampus was fractionated in a 10-31% linear sucrose gradient. The size of the mRNA required to express the inwardly rectifying current was estimated to be around 4 kb. 7. In conclusion, the expressed current is an inwardly rectifying potassium current. The canine hippocampal mRNA should be an excellent source for expression-cloning of the inward rectifier channel.