Whole cell patch clamp recordings were employed to characterize the electrophysiological properties of layer 5 pyramidal cells in slices of the prefrontal cortex (Area 46) of the rhesus monkey. Four electrophysiologically distinct cell types were discriminated based on distinctive repetitive action potential (AP) firing patterns and single AP characteristics: regular-spiking slowly-adapting type-1 cells (RS1; 62%), regular spiking slowly-adapting type-2 cells (RS2; 18%), regular spiking fast-adapting cells (FA; 15%), and intrinsically bursting cells (IB; 5%). Cell types did not differ with regard to their location in layer 5 or in their dendritic morphology. In RS1 cells, AP threshold and amplitude did not change significantly during a 2 second spike train, whereas in RS2 and FA cells AP threshold increased significantly and AP amplitude decreased significantly during the train. In FA cells, complete adaptation of AP firing was observed within 600 ms. IB cells displayed an all-or-none burst of 3-6 APs, followed by RS1-type firing behavior. RS1 cells could be further subdivided into 3 subtypes. Low threshold spiking (LTS) RS1 cells exhibited an initial doublet at the onset of a spike train and a prominent depolarizing afterpotential (DAP); intermediate RS1 cells (IM) exhibited a DAP, but no initial doublet, and non-LTS RS1 cells exhibited neither a DAP nor an initial doublet. RS2 and FA cells did not exhibit DAPs or initial doublets. The distinctive firing patterns of these diverse layer 5 pyramidal cells may reflect different roles played by these cells in the mediation of subcortical neuronal activity by the dorsolateral PFC.