We present field potential and multiple single-unit data simultaneously recorded from somatosensory cortex of the anesthetized rat using a multi- site silicon electrode. We find that although single-units fire spontaneously only during troughs of the low-frequency field potential, there is high variability in the timing of their firing. Viewing our results in the context of previous anatomical, physiological, and modeling studies of cerebral cortex, we suggest that local field potentials are better characterized as a recording of local synaptic currents rather than a recording of the superposition of spike activity in populations of neurons. If true, then our data show that similar circuit-level synaptic inputs do not predict the timing of spiking output by the neurons receiving those inputs. These results have important implications for theories of cerebral cortical network function and information coding based on the interpretation of coherence in recorded field potentials. (C) 2000 Elsevier Science B.V. All rights reserved.