Transsynaptic transport of conjugates of wheat germ agglutinin with horseradish peroxidase (WGA-HRP) has been studied in rats and cats. WGA-HRP was injected into a muscle nerve from which it was first transported to motoneurones and along sensory fibres, and secondarily to interneurones. More extensive labelling of interneurones occurred in preparations with only relevant ventral roots intact than in preparations with only dorsal roots intact, which indicates that the transsynaptic transport of WGA-HRP is primarily in the retrograde direction, i.e. from motoneurones to interneurones. Such a transport appeared to be considerably enhanced by neuronal activity.