THERE are few estimates of the rates at which spiking neurons transmit information(1-5), and none for synapses transmitting graded signals, We have measured the rates at which blowfly (Calliphora vicina) photoreceptors transmit information through chemical synapses to large monopolar cells (LMCs), The graded responses of these non-spiking cells transmit as much as 1,650 bits per second, five times the highest rates measured in spiking neurons(3). The widespread occurrence of non-spiking neurons in sensory systems could well reflect this superior performance. Comparing transmission rates in pre- and postsynaptic cells, we estimate that each synaptic active zone transmits similar to 50 bits s(-1). This estimate assumes statistical independence of the noise generated at active zones and makes use of a detailed morphometric analysis of photoreceptor-LMC synapses(6,7). These measurements provide a benchmark for quantifying the performance of synaptic mechanisms and for understanding the limitations that synaptic transmission and spike coding place upon neural computation.