ABSTRACT. Periodic breathing cycle duration (PCD),PB, a respiratory pattern consisting of alternating respiratory pauses and breaths, has been studied extensively in infants during the last decade. PB comprises <2% of the respiratory pattern in most normal infants during the first months of life (1-3), and occurs more frequently and in higher amounts in preterm infants (4-8). PB may therefore reflect an immaturity of developing respiratory control system elements which does not normally indicate any abnormalities or pathology; however, elevated amounts of PB have been reported in some infants "near-miss" for SIDS and in some SIDS victims (9-1 3).Although most studies of PB in infants have been limited to observations on the incidence or amount of PB in normal and high-risk groups, relatively few investigations have focused on the mechanisms underlying PB. Recently, Barrington et al. (14) described a decrease in PCD in te>m infants during thefirst 3 mo of life. PCD was calculated for 2-10 cycles/PB episode and was definid as the time interval between the initial breath of one breathingjrespiratory pause cycle and the initial breath of the