Cortical local field potentials (LFPs) are modulated in parallel with single neuron discharge, but the information they carry is often unclear. Multi-electrode recordings of both LFPs and single neuron activities were made in motor cortex as monkeys performed a delayed pointing task in which the probability of the moment of signal occurrence, and thus movement execution, was manipulated. A large positive LFP component (P1) appeared immediately preceding movement onset only under conditions of low probability, that is, when a response signal was weakly expected. The amplitude of P1 was much smaller when probability of signal occurrence was high, or when the same movement was self-paced. Although P1 has been described as being linked to the descending motor signal, we found that it was more closely associated with the processing of movement-related information than with the ultimate motor command. Its timing did not bear a fixed relationship with movement onset and its frequency of occurrence in each monkey varied in parallel with each animal's overall performance and the percentage of context-related "pre-processing" neurons encountered.