Lamina I of the spinal dorsal horn plays an important role in the processing and relay of nociceptive information. Signal processing depends, in part, on neuronal membrane properties. Intrinsic membrane properties of lamina I neurons were therefore investigated using whole cell patch clamp recordings in a slice preparation of adult rat spinal cord. Based on responses to somatic current injection, four cell types were identified: tonic, which fire comparatively slowly but continuously throughout stimulation; phasic, which fire a high frequency burst of variable duration; delayed onset, which fire irregularly and with a marked delay to the first spike; and single spike, which typically fire only one action potential even when strongly depolarised. Classification by spiking pattern was further refined by identification of characteristic stimulus-response curves and quantification of several response parameters. Objectivity of the classification was confirmed by cluster analysis. Responses to stimulus trains and synaptic input as well as the kinetics of spontaneous synaptic events revealed differences in the signal processing characteristics of the cell types: tonic and delayed onset cells appeared to act predominantly as integrators whereas phasic and single spike cells acted as coincidence detectors. Intracellular labelling revealed a significant correlation between morphological and physiological cell types: tonic cells were typically fusiform, phasic cells were pyramidal, and delayed onset and single spike cells were multipolar. Thus, there are multiple physiological cells types in lamina I with specific morphological correlates and distinctive signal processing characteristics that confer significant differences in the transduction of input into spike trains.