Summation of the potassium conductance (GK) changes underlying the spike afterhyperpolarization (AHP) has been studied in cat spinal motoneurones. Cells were directly activated by one to five short current pulses at constant rate, each evoking an action potential. The analysis was restricted to cells displaying an approximately exponential decay of the AHP conductance. In these neurones the AHP conductances given by successive spikes were found to summate in a non-linear manner. This nonlinear summation seemed well described by a neurone model based on modified Hodgkin-Huxley equations. From the model equations the total AHP conductance in motoneurones could be calculated from values of GK measured experimentally at different times during the summation process. Adaptation and steady-state firing in motoneurones are assumed to be governed by summation of AHP conductance. The same model was then utilized for simulating neuronal repetitive firing in response to current steps. Such simulations were performed after substitution of the model parameters with values measured in individual motoneurones which had also been fired repetitively by intracellular injection of long-lasting current steps. The amount of adaptation and the shape and slopes of the steady-state frequency-to-current relation were found to coincide in the model and in the corresponding motoneurones.