This experiment was designed to simulate experimentally the specific parameters of geomagnetic activity that evoke epileptic seizures. The numbers of overt limbic seizures (rearing, paroxysmal forelimb clonus and falling) in a population of epileptic rats were recorded nightly for 65 successive days between 0200 and 0400 h during red light conditions. On some nights an experimental 7 Hz magnetic field whose magnitudes shifted in successive steps from zero to approximately 50 nT every 3 min was presented. The partial regression coefficients from the analysis indicated that either the presence of the 'synthetic' geomagnetic activity or increased magnitudes of the daily, natural geomagnetic activity (regional range approximately 10-70 nT) during the observational period significantly (P < 0.05) increased the proportion of nightly seizures. The effect sizes (6-8%) for both magnetic sources were comparable and additive. Concerted efforts to experimentally simulate the temporal profiles of geomagnetic activity may help reveal the neuromechanisms by which biobehavioral changes during geomagnetic perturbations occur within susceptible populations.