1. Intracellular recordings and quantal analysis of synaptic transmission were made at neuromuscular junctions receiving stable convergent innervation in reinnervated rat lumbrical muscles, following recovery from chronic nerve conduction block. The polyneuronally innervated motor endplates (pi-junctions) were identified by vital staining of lateral plantar nerve (LPN) and sural nerve (SN) motor terminals, using the activity-dependent staining properties of the aminostyryl dyes RH414 and FM1-43, respectively. 2. Endplate depolarisation and quantal content per unit area varied by more than a factor of ten ( approximately 0.1-1. 4 quanta microm-2) between fibres. However, the stable pi-junctions produced nearly equivalent endplate depolarisations and quantal content per unit area, suggesting that synaptic strengths were co-regulated at these motor endplates. Quantal content per unit area was also independent of the size of individual synaptic inputs, or whether one, both or neither input was judged sufficient to produce suprathreshold or subthreshold endplate depolarisations. 3. Simultaneous excitation of convergent LPN and SN inputs from some pi-junctions resulted in profound non-linear summation, and in some cases complete occlusion of the response of the smaller input. The amplitude of the smaller, test responses recovered with a time constant of 2.1 +/- 0.5 ms (mean +/- s.e.m.) on varying the interval between paired stimuli, of similar order to the time constant of repolarisation of the conditioning endplate potential. 4. The data show that it is not necessary for a motor nerve terminal to occupy most of an endplate, or to produce a suprathreshold response in order to become stable. The occlusion of linear summation, similar to that described previously at polyneuronal junctions in neonates, suggests that convergent inputs comprising interdigitated synaptic boutons evoke self-contained synaptic responses at endplates, and that these are non-co-operative with respect to overall endplate depolarisation or safety margin for synaptic transmission.