The cortex provides a massive input to striatal neurons but some of its properties make it extremely difficult to study single synapses between cortical fibres and striatal cells. For instance, each striatal cell may receive in the order of 5,000 cortical inputs, from an almost identical number of cortical neurons. However, the neighbouring striatal cells are extremely unlikely to respond to the same set of neurons since the likelihood of sharing more than about 1% of the inputs is vanishingly small (Kincaid et al., 1998). We therefore have developed methods to culture cortical cells from green fluorescent protein expressing mouse embryos together with striatal neurons from wild type mice of the same developmental stage ( E14.5 from QBM Cell Science, Ottawa, Canada). It is then possible to record from pairs of neurons in culture and to be sure that one is cortical and the other striatal. We have used conventional whole cell patch clamp recording methods in cultures (10-28 days in vitro) recorded in artificial media of composition (NaCl 136; KCl 5; MgCl 1; CaCl 2.5; Hepes buffer 10; Glucose 10, all mM) at 36 oC. The glass micropipettes had a resistance of 6-12MΩ internal solution (NaCl 8; K Gluconate 132; MgATP 2; NaGTP 0.4; KCl 6; Hepes buffer 10, all mM). There are some striato-cortical connections (5/18 paired recordings), all are inhibitory in nature (reversing at approximately -40mV). The currents are 129.8±41pA at -80mV and have a latency of onset of 1.9±0.23ms (mean± S.D. n=5). The cortico-striatal connections have been characterized by not being reversed near the chloride equilibrium potential but reversing much closer to 0mV. Thus we are able to distinguish these excitatory synaptic currents and estimate that they are 79.3±20.7pA at -80mV with a latency of 2.4±0.5 ms to the onset of the response (n=6). The range is large in these first few pairs, but we have not yet been able to reconstruct the anatomical details of the connections, nor to estimate the quantal characteristics of the connections we have seen. We will compare the currents generated by individual synaptic contacts between identified cortical and striatal cells which we measure, with experiments where the currents are generated by direct electrical stimulation, or by the uncaging of glutamate, close to a single spine in slices of the corticostriatal system from mice. The use of cortical and striatal cells in culture is providing us with an excellent tool to further understand the electrophysiology of this important synaptic input.