The main neuronal population of the striatum is composed of the medium spiny neurones (MSNs). In fact several sub-populations of MSNs can be distinguished according to the striatal compartment (striosomes and matrix) to which they belong, their afferents and their sites of projection, their biochemical markers and their morphologies. However, these cells are generally described as an electrophysiological homogeneous population. Using brain slices from the rat and whole cell patch clamp recordings, we show that at P(15) 28% of the MSNs display a spike frequency adaptation. While the mean frequency adaptation ratio for non-adapting cells was 1.07+/-0.01 it reached 2.66+/-0.09 in adapting MSNs and the incidence of this frequency adaptation phenotype appeared to be stable during post-natal development. Single-cell RT-PCR analysis of mRNAs for mu opioid receptors, enkephalin and substance P precursors suggested that adapting MSNs are present in both striatal compartments as well as in the direct and indirect pathways of the matrix. Adapting neurones were also distinguished from non-adapting cells by a lower membrane time constant, a higher AP threshold, a reduced delay to the first spike and a higher initial firing rate. Micro-domains differing by their magnitude of adaptation could be distinguished within the spike frequency adaptation process.A subgroup of MSNs exists, showing a marked spike frequency adaptation together with other distinct properties, such as shorter delay to first spike and membrane time constant, and higher initial frequency and action potential threshold. In conclusion, when strong cortical inputs are delivered in coincidence, adapting MSNs could not only transmit faster the first AP but also exert a sort of cutoff of the transmission due to their spike frequency adaptation process.