1. Experiments, using electrophysiological recording techniques, were carried out to confirm the report that prostaglandins affect spinal reflexes in cats.2. Ventral root potentials evoked by stimulation of the ipsilateral dorsal root were recorded. Those with a latent period corresponding to a monosynaptic pathway were used primarily in this investigation.3. Prostaglandin E(1) (3.5-17.8 mug/kg) injected into the aorta reduced the amplitude of monosynaptic responses in six out of ten cats. The effect began about 15 min after injection and lasted over 3 hr. In one cat E(1) caused potentiation of the reflex and in three cats there was no effect.4. Prostaglandin F(1alpha) (2.4-3.5 mug/kg) inhibited the monosynaptic response in four cats but in one of these a subsequent large dose (19.6 mug/kg) greatly potentiated the reflex.5. Prostaglandin F(2alpha) (1.4-17.8 mug/kg) injected into the aorta was followed by significant but variable changes in monosynaptic response. In one experiment an intravenous injection (30.3 mug/kg) was followed by a long-lasting potentiation of the reflex response.6. It is concluded that prostaglandins, two of which have previously been identified in cat brain extracts, have pronounced and long-lasting effects on monosynaptic spinal reflexes.