1. From denervation studies the trophic influence of the motor nerve on the muscle cell is well documented while little is known about the influence of the muscle on the nerve. Sectioning the axon invariably destroys the nerve terminals and produces nerve degeneration products which themselves may affect nerve and muscle properties. With regard to those difficulties we believe that the botulinal neurotoxins (BoTx) are valuable complements to denervation since they selectively interrupt impulse transmission across the synapse without damaging its morphology. 2. Paralysis of mouse or rat skeletal muscle in vivo with BoTx type A causes marked growth of motor nerve terminals. The sprouting terminals are rich in large dense-core synaptic vesicles containing various neuropeptides and they spontaneously release large quanta of ACh. Thus, it appears that paralysis by BoTx is a strong stimulus for motor nerve growth and the delivery of "trophic" substances to the nerve terminals. 3. Postsynaptically, in extrajunctional areas, paralysis by BoTx induces all the changes observed following denervation, i.e. atrophy, appearance of extra-junctional ACh receptors, TTX-resistant action potentials, a fall of resting membrane potential, fibrillation potentials and the disappearance of extrajunctional acetylcholinesterase activity. Endplate properties are, however, largely maintained. 4. BoTx blockade delays and prevents the retraction of polyneuronal innervation and motoneurone death during development. This supports the suggestion that the paralysed muscle secretes factors essential for growth and for the survival of motoneurones. 5. Like denervated muscle, BoTx paralysed ones, express a high endocytotic activity restricted to a segment in the endplate region.(ABSTRACT TRUNCATED AT 250 WORDS)