AIM: To study the effect of postnatal age, weight, sex, and chronological order on time to acidosis or fall in blood pressure, and isoelectric EEG in hypoxic piglets. SUBJECTS: Twentyseven piglets, aged 2-5 days, in general anesthesia with halothane, fentanyl, and pancuronium. Two piglets were studied each day. INTERVENTION: Hypoxia (FiO2 6%) and inhaled CO2 (paCO2 7-8 kPa). Time to BE lower than -25 mmol/L or a drop in mean arterial blood pressure <25 mm Hg (hypoxia time) and time to isoelectric EEG (isoEEG). RESULTS: There was an inverse linear relation between age and hypoxia time (p= 0.007). Piglets in the afternoon had longer time to isoEEG (afternoon 30,4±7,9 min vs. morning 17,2± 12,3 min., p=0,003). No other differences were found. CONCLUSIONS: This study demonstrates the effect of postnatal age on hypoxic resistance in piglets. The increased durability of EEG in piglets studied in the afternoon was surprising.