Abstract. We examine the influence of dendritic load on the firing dynamics of a spatially extended leaky integrate-and-fire (LIF) neuron that explicitly includes spiking dynamics. We obtain an exact analytical solution for this model and use it to derive a return map that completely captures the dynamics of the system. Using the map, we find that dendritic properties can significantly change the firing dynamics of the system. Under certain conditions, the addition of the dendrite can change the LIF model from type 1 excitability to type 2 excitability and induce bistability between periodic firing and the quiescent state. We identify the mechanism that causes the periodic behavior in the bistable regime as somatodendritic ping-pong. Furthermore, we use the return map to fully explore the model parameter space in order to find regions where this bistable behavior occurs. We then give physical interpretations of the dependence of the bistable behavior on model parameters. Finally, we demonstrate that the simpler two-compartment model displays qualitatively similar dynamics to the more complicated ball-and-stick model. 1. Introduction. Neurons are spatially extensive and heterogeneous. They typically consist of a dendritic tree, a soma (cell body), and an axon. The type of model one uses to represent a neuron depends upon a balance between mathematical tractability and biological realism and on the issue being addressed. A common technique in neuronal modeling is to represent the neuron as a single-compartment object that ignores the spatial anatomy of the cell, e.g., [11,25,26]. Although this simplification allows for greater mathematical tractability and computational efficiency, many neurons are not electrotonically compact. Therefore, singlecompartment models cannot be expected to capture the full spectrum of electrical behavior of neurons. Dendrites can have substantial effects on the dynamics of individual neurons. For example, the architecture and ionic channel density of a dendritic tree can alter the firing pattern and encoding properties of a neuronal oscillator [19,21,24], while the additional electrical load due to the dendrite can alter firing frequency [36,38]. For a full understanding of this behavior, more detailed models are required.