The effect of systemic administration of naloxone on transmission in hindlimb reflex pathways was investigated in acute low spinal cats by conditioning monosynaptic reflexes. A marked enhancement of excitatory effects from cutaneous, joint, group II and III muscle afferents was observed in posterior biceps and semitendinosus motoneurones in 4 out of 6 experiments. In contrast, inhibitory synaptic effects in gastrocnemius and soleus motoneurones were not enhanced except weakly in one experiment. The effects of naloxone are different from those observed after spinal cord lesions interrupting axons of a previously described group of upper lumbar propriospinal neurones, which tonically suppress reflex transmission in the acute low spinal state. It is postulated that the suppression exerted by this group of neurones is not dependent on endogenous opioid peptides.