The properties of K currents of pineal cells were studied using the whole-cell variant of the patch-clamp technique. The total K current could be separated in two distinct components: a fast, transient current (It) and a slow current (Is). The activation threshold of It was at -35 to -30 mV. On depolarization to +50 mV it reaches a peak in 2-3 ms and inactivates almost completely in 50 ms. Half steady state inactivation occurs at -45 mV. Inactivation of It is voltage-dependent and is well fitted by single exponentials with time constants between 17.2 ms at +50 mV and 27.2 ms at -10 mV. Inactivation is removed with time and the recovery period shortened by membrane hyperpolarization. The slow K current has a threshold at -20 to -15 mV. It reaches a maximum in about 30-40 ms and inactivates slightly, to about 80% of the peak value at the end of pulses lasting 200 ms. With 80 mM external K, tail currents recorded after short (1-2 ms) depolarizations were about 2.5 times faster than the tails recorded at the end of 50 ms pulses. The fast tails were removed by depolarizing prepulses but the slow tails remained unaltered. Thus, the fast and slow tails are probably a reflection of the closing of the transient and slow K channels. The transient K current of pineal cells has general characteristics similar to transient currents recorded in non-secretory cells, but also has particular kinetic properties.