We describe a vibrating probe system for measuring relatively steady electrical current densities near individual living cells. It has a signal-to-noise ratio at least 100 times greater than previously available techniques. Thus it can be used to detect current densities as small as 10 nA/cm(2) in serum when a 30-microm diameter probe is vibrated at 200 Hz between two points 30 microm apart, and the amplifier's time constant is set at 10 s. Moreover, it should be generally insensitive to interference by concentration gradients. It has been first used to reveal and study 100-s long current pulses which developing fucoid embryos drive through themselves.