The effects on lumbar motoneurons of thoracic cord stimulation were investigated in the neonatal rat hemisected spinal cord in vitro using intracellular recording. Four responses were evoked--a fast, excitatory postsynaptic potential, a second component to the fast excitatory postsynaptic potential, a fast inhibitory postsynaptic potential and a slow excitatory postsynaptic potential. The fast (CNQX-sensitive) excitatory postsynaptic potential was probably monosynaptic, was blocked by CNQX, (10 microM) and showed a frequency-dependent run-down at stimulation frequencies between 0.1 and 1 Hz. A slower component to the fast excitatory postsynaptic potential ((+-)-2-amino-5- phosphono-valeric acid-sensitive excitatory postsynaptic potential) was blocked by (+-)-2-amino-5-phosphonovaleric acid (50 microM). Following fast excitatory postsynaptic potential blockade with both CNQX and (+-)-2-amino-5-phosphonovaleric acid, a fast inhibitory postsynaptic potential was revealed. This reversed at a membrane potential close to resting and was incompletely blocked by either bicuculline (30 microM) or strychnine (10 microM). The slow excitatory postsynaptic potential was a delayed depolarization associated with a small increase in input resistance (20%) and was insensitive to block by CNQX and/or (+/-)-2-amino-5-phosphonovaleric acid. It increased in amplitude on membrane depolarization and decreased on hyperpolarization and was potentiated by cocaine (3 microM) and citalopram (0.1 microM), but not by desipramine (5 microM). The slow excitatory postsynaptic potential was blocked by ketanserin (1 microM) and by LY 53857 (1 microM). It is concluded that a non-glutamatergic transmitter is involved in generating the slow excitatory postsynaptic potential possibly 5-hydroxytryptamine acting at 5-hydroxytryptamine 2 receptors.