A technique is presented for the measurement of fluctuations in processing demands during spontaneous speech. The technique consists of the analysis of errors on a secondary tracking task. Data are presented from illustrative samples of spontaneous speech; thus, evidence was found to suggest that one level of planning in speech is clauses that contain a single main verb. Evidence was also obtained that there is an increase in processing demands at the gap in subject relative and object relative clauses. It is concluded that the secondary tracking task is a useful technique that could be extended to studies of reading and speech comprehension.