The outward current was analysed in the rapidly adapting stretch receptor neuron of the crayfish Pacifastacus leniusculus with a two-micropipette potential clamp technique and K(+)-selective microelectrodes in an attempt to establish if the properties of this current could explain the difference in adaptive behaviour compared to the slowly adapting receptor. A fast activating outward current carried by K+ was revealed. The time constant of activation(tau n) was dependent on potential and had a mean value of 0.5 ms at potential steps to 0 mV. Activation followed a second-order process according to the Hodgkin-Huxley model. The potential dependence of activation (n infinity) followed by a sigmoid curve n infinity = 1/(1 + exp/[(E - En)/a]) with a half maximal activation potential En = -44 mV and a = -13 mV. When long pulses were applied the outward potassium current decreased with two time constants, one that was potential independent (0.2 s) and one that was potential dependent (2-8 s). The latter could be explained by accumulation of K+ in the extracellular space of the neuron. The potential dependence of inactivation followed a sigmoid function infinity = 1/(1 + exp[(E - Ek)/+a]) with Ek = -36 mV and a = 13 mV. The inactivation properties are different from those of the classical fast transient (IA) current. The transport system for the outward potassium current during depolarizing potential steps in the rapidly adapting stretch receptor is similar to the current found in the slowly adapting receptor neuron. However, the activation is faster and seems to occur at potentials more negative than in the slowly adapting receptor. These differences can contribute to but not entirely explain the difference in adaptive behaviour between the slowly and rapidly adapting receptor.