1. The neurons of the retina have electrical properties that are different from those of most of the other neurons of the central nervous system. To identify the voltage-gated ion channels found in the retina, we screened mouse retinal cDNA libraries with oligonucleotide probes homologous to the mammalian K+ channel MBK1 (Kv1.1) and ligated two partial clones to produce a full-length clone with no significant differences from MBK1. 2. Expression of MBK1 mRNA was determined by RNAse protection. MBK1 mRNA was detected in retinal RNA and was also detected in brain, liver, and heart RNAs. 3. We transcribed the full-length clone, injected it into oocytes of Xenopus laevis, and measured the membrane currents 2 to 6 days later. Depolarization from a holding voltage of -90mV induced a slowly activated outward current with a peak value as large as 20 microA. The current inactivated very slowly with a single exponential time course [mean time constant, 6.5 +/- 0.4 sec (SEM) for activation voltage of -10mV]. 4. The outward current was reduced to half-maximal by 0.42 mM tetraethylammonium, 1.1 mM 4-aminopyridine, and 3.2 mM Ba2+ but was not significantly attenuated by Co2+ (1 mM). 5. The reversal potential (measured with tail currents) changed by 53mV per decade change of [K+] from 1 to 77 mM. 6. The voltage for half-maximal activation of the conductance was -26.6mV (+/- 1.7mV), and the voltage required for an e-fold increase in conductance was 6.9mV (+/- 0.5mV). 7. Thus, the mRNA for MBK1 found in the mouse retina causes the expression of a voltage-dependent K+ current which has properties suitable for may retinal neurons.