Ventilation was monitored in 10 human subjects during spontaneous conversational speech to determine whether oscillatory patterns in vocal activity were correlated with oscillatory patterns in ventilation. The 10 subjects were studied as five pairs (or dyads), and spontaneous conversation occurred within each dyad. Patterns with cycle times ranging from 16 to 512 s were studied. Of the 10 subjects in this study, 1 subject showed a very striking pattern of mutual entrainment between low-frequency oscillations in ventilation and in vocal activity, 5 subjects showed somewhat weaker coordination between oscillations in ventilation and vocal activity, and the remaining 4 subjects showed little or no coupling between ventilation and vocal activity. Mutual entrainment between rhythms in ventilation and vocal activity can occur, but this study suggests that there may be great differences among individuals in the degree to which ventilation covaries with vocal activity in spontaneous conversational speech. We hypothesize that degree of entrainment is affected both by the strength of any spontaneous ventilatory patterns and by the extent to which any individual's spontaneous ventilatory pattern matches that of his conversational partner.