The effects of nano particles of CuO on voltage-dependent potassium currents were studied in acutely isolated CA1 pyramidal neurons of rat hippocampus using the whole-cell patch-clamp techniques. Nano particles of CuO had small effects on transient outward potassium current (IA, no statistical significance) and mainly inhibited delayed rectifier potassium current (IK) in the concentration of 5 × 10−5 g/mL. Nano particles of CuO didn't shift the steady-state activation curve of IK and IA but negatively shifted the inactivation curve of IK. The effects on inactivation curve of IA had no statistical significance. These results suggested that blockades of K+ currents by nano particles of CuO could be preferential for Ik for the first time. This may interfere with the normal function of nerve cells. © 2008 Wiley Periodicals, Inc. Environ Toxicol, 2009.