1. The dynamic clamp was used to create reciprocally inhibitory two-cell circuits from pairs of pharmacologically isolated gastric mill neurons of the stomatogastric ganglion of the crab, Cancer borealis. 2. We used this system to study how systematic alterations in intrinsic and synaptic parameters affected the network behavior. This has previously only been possible in purely computational systems. 3. In the absence of additional hyperpolarization-activated inward current (IH), stable half-center oscillatory behavior was not observed. In the presence of additional IH, a variety of circuit dynamics, including stable half-center oscillatory activity, was produced. 4. Stable half-center behavior requires that the synaptic threshold lie within the voltage envelope of the slow wave oscillation. 5. Changes in the synaptic threshold produce dramatic changes in half-center period. As predicted by previous theoretical work, when the synaptic threshold is depolarized, the period first increases and then decreases in a characteristic inverted U-shaped relationship. Analysis of the currents responsible for the transition between the active and inhibited neurons shows that the mechanism of oscillation changes as the synaptic threshold is varied. 6. Increasing the time constant and the conductance of the inhibitory synaptic current increased the period of the half-center oscillator. 7. Increasing the conductance of IH or changing the voltage dependence of IH can either increase or decrease network period, depending on the initial mode of network oscillation. A depolarization of the activation curve causes the network to respond in a similar fashion as increasing the conductance of IH. 8. Many neuromodulatory substances are known to alter synaptic strength and the conductance and voltage dependence of IH, parameters we studied with the dynamic clamp. To understand the response of the network to modulation of a single parameter, it is necessary to understand the nature of the altered conductance and how it interacts with the other conductances in the system.