An analysis was made of effects of 0.1-1.0 mg/kg 4-aminopyridine (4-AP) i.v. on excitatory and inhibitory spinal reflex pathways in lightly anaesthetized or decerebrated cats. The effects appeared within the first minutes of the injection, reached maximum after about 10-15 min and remained stable during at least several hours. 4-AP enhanced the following synaptic actions on motoneurones: monosynaptic excitation from Ia afferents and descending tracts, disynaptic and polysynaptic excitation from group Ib, group II, cutaneous and high threshold muscle afferents, disynaptic inhibition from Ia and Ib afferents and recurrent and polysynaptic inhibition from different afferents. 4-AP also increased primary afferent depolarization and excitation of ascending tract cells by peripheral stimuli. In the case of the disynaptic inhibitory pathways it has been shown that 4-AP may enhance the excitation of the interposed interneurones but it also increases the action of these interneurones on the motoneurones; monosynaptic inhibition evoked in motoneurones by electrical stimulation of the axons of the inhibitory interneurones was used as a test response in these experiments. No indications were found of direct effects of 4-AP on excitability of afferent fibres or motoneurones to electrical stimuli. No systematic changes were either found in the membrane potential of motoneurones or in the duration of action potentials of these neurones or primary afferents. It is therefore concluded that small doses of 4-AP enhance synaptic transmission in the spinal cord by an action at a presynaptic level.