1. Spontaneous synaptic potentials were recorded in motoneurons grown in organotypic slice cultures of embryonic rat spinal cord. In 71 of 85 cells these potentials appeared without obvious temporal structure (random patterns); in the remaining 14 cells they appeared in bursts (rhythmic patterns). 2. Random activity patterns could be converted into rhythmic patterns by treating the cultures with strychnine, bicuculline, or both. The excitatory amino acid N-methyl-D-aspartate (NMDA) transiently increased the rate of spontaneous synaptic activity without inducing rhythmic patterns. The NMDA antagonist 7-chloro-kynurenate reduced the burst rate while leaving the burst length unchanged in rhythmic patterns. In random patterns it reduced the rate of spontaneous synaptic activity by 68%. 3. Histograms of interevent times of the random patterns were best fitted by the sum of two expontentials, suggesting that the random type of activity could not be described simply as a Poisson process but involved at least one additional mechanism. 4. Rhythmic patterns consisted of bursts of activity with a mean burst length of 2.2 s that were separated by interburst intervals with a mean length of 6.6 s. Within the bursts autocorrelograms revealed regular oscillations with a mean period of 226 ms in 6 of 11 experiments with rhythmic patterns. The period showed little variation between individual experiments (202-288 ms). In random patterns no oscillations were detected. 5. Within the spontaneous bursts the excitatory postsynaptic potentials (EPSPs) progressively declined in amplitude. A corresponding depression of EPSPs was observed when trains of electrical stimuli were applied at 5 Hz to the dorsal horns of the spinal cord slices.(ABSTRACT TRUNCATED AT 250 WORDS)