Graded synaptic transmission between spiking motoneurons of the pyloric group was studied in the stomatogastric ganglion of the spiny lobster, Panulirus interruptus. Intracellular microelectrodes were placed in the cell bodies of both pre- and postsynaptic neurons. Graded synaptic transmission was found between all tested cell pairs that were known to display spike-evoked synaptic transmission, including PD to LP, PD to PE, PD to PL, PL to LP, and LP to PD. Graded synaptic transmission was effective below the threshold for spikes. Thus, it was possible to study the influence of graded synaptic transmission in normally active ganglia without blockage of spikes by tetrodotoxin. PD and LP neurons that were known to produce spike-evoked inhibitory postsynaptic potentials (IPSPs) were also capable of producing inhibitory effects on postsynaptic cells below the threshold for spikes. When tetrodotoxin (TTX) was used to eliminate both spikes and endogenous membrane oscillations, depolarization of presynaptic neurons produced hyperpolarization of postsynaptic cells. The presynaptic response to a current step usually showed a small early peak and a maintained, slightly lower plateau. The postsynaptic response had a delay, then a rise to a pronounced peak, and a roughly exponential decline to a maintained plateau. There was a presynaptic voltage threshold for any postsynaptic response; beyond the threshold, both pre- and postsynaptic peak and plateau responses increased with increasing current. PD neurons normally are depolarized beyond their release threshold in tetrodotoxin and, thus, released transmitter tonically for the many-hour duration of these experiments. Chemical, tonic synaptic transmission, here called graded synaptic transmission, was demonstrated by the presence of the following criteria: 1) reversal in sign of the postsynaptic response, 2) synaptic delay, 3) reversal potential, 4) postsynaptic conductance increase, 5) graded and reversible block by reduction of external Ca2+, and 6) specific graded block of the LP-to-PD synapse without effect on the PD-to-LP synapse by less than 10 microM picrotoxin added to the bathing medium.