Effects of different patterns of presynaptic stimulation upon release of leuteinizing hormone releasing hormone (LHRH) were studied by monitoring LHRH-induced slow currents from individual postsynaptic neurons in bullfrog sympathetic ganglia. LHRH-mediated synaptic currents in ganglionic B and C neurons were recorded by a single- electrode voltage-clamp technique. Using continuous stimulation, release increased with frequency between 2 and 20 Hz, then declined. Though bursts of stimuli always evoked more release than continuous stimuli of the same average frequency, they were invariably less effective than continuous stimulation at the intraburstal frequency. This demonstrates that frequency, not bursting structure, governs peptide release. The dependence of release upon stimulus frequency was altered when extracellular Ca2+ concentration was changed, implying that release does not depend intrinsically upon stimulation frequency, but simply on the availability of Ca2+.