Specific types of interneurones have distinct functional roles for cortical processing. One of the defining characteristics in this respect is their anatomical embedding within local networks. Unfortunately, single unit recordings, as used in freely behaving animals, currently do not allow the anatomical identification of the recorded cells. However, identification of the cell type recorded is prerequisite for our understanding of their function in the network. The aim of the current project is to develop criteria for a classification of interneurones in recordings from freely behaving animals. After developing the methodology to characterise the extracellular waveshape of anatomically identified cells in vitro (cf. Ganter et al., SfN 2004) we use these results to analyse data from cells in stratum oriens of CA1 in freely behaving rats. Amongst the different types of interneurones (putative) O-LM cells could be distinguished in vitro by a late component lasting for about 10 ms possibly reflecting a dendrit...