Data about the use of ranitidine in the early postnatal period are lacking. In this study, 30 term newborn infants < 2 days old with bleeding erosions in their upper gastrointestinal tracts were treated with ranitidine by continuous i.v. infusion (0.2 mg/kg/h) for 48 h and thereafter by mouth (5 mg/kg b.i.d.) for 1 month. Mean gastric pH (SD) rose from 4.27 (1.62) to 5.70 (0.95) during i.v. infusion; after oral therapy it was still 5.55 (1.25). Serum ranitidine concentrations were 642.4 (376.5) and 321.5 (368.2) ng/ml after i.v. and oral therapy, respectively, with wide interindividual variations; the correlation between serum ranitidine and gastric pH was found to be weak. No untoward effect was observed either on the cardiorespiratory rate or on creatinine and aminotransferase values. Mean serum prolactin concentration after i.v. therapy was found to be lower, although within the reference range, than in control infants; no significant correlation was observed between serum ranitidine and prolactin concentrations. From these data, a < 0.2 mg/kg/h rate seems to be advisable for continuous ranitidine infusion in neonates, whereas the 5 mg/kg b.i.d. regimen could be considered adequate for oral therapy.