The aim of this quasi-experimental study was to examine the effects of maternal pethidine during labour on the developing breast feeding behaviour in infants in the first 2 h after birth compared with infants not exposed to pethidine. Forty-four healthy infants were observed immediately after birth. They were placed skin-to-skin on their mothers' chests. The development of mouth and sucking movements as well as rooting behaviour and state of sleep/wakefulness were noted. The observer was blind as to the pain relief the mother had received during labour. Of the 44 mothers 18 had received pethidine. The main findings were that infants exposed to pethidine had delayed and depressed sucking and rooting behaviour. In addition, a smaller proportion of infants exposed to pethidine started to suckle the breast. Rooting movements which are expected to be vigorous at 30 min after birth were affected both by administration of pethidine and a longer second stage of labour. It is suggested that the differences found in sucking behaviour may be a central effect of pethidine. Depression of rooting movements in the pethidine group may be caused by exhaustion due to a longer second stage of labour and administration of pethidine. It is recommended that pethidine-exposed mother-infant couples stay together after birth long enough to enable the infant to make the choice to attach or not to attach to the nipple without the forceful helping hand of the health staff.