Effects of age and magnesium ions on oxygen toxicity in the neonate chicken

White Leghorn male chicks, 2-6 days and 4 weeks old, were injected with MgCl2 (2 mM Mg/kg body weight) and exposed to 100% O2 at 75 lb/in2 (approximately 5 atm). Compared to saline-injected controls, average time to onset of convulsions was delayed 37% in the younger group with no significant difference in the protective effect during days 2-6. The delay in onset of convulsions was 66% in the 4-week-old group. The protective action of Mg++ in the chick agrees closely with that reported for the mature rat. Chicks treated with Mg++ did not appear depressed. Susceptibility to hyperbaric O2 convulsions increased uniformly between the 2nd and 6th day, with time to onset of convulsions decreasing by 1.4 min/day from the 19.3 +/- 1.01 (SE) min observed at day 2 for saline controls. Between the 6th and 28th day, susceptibility was relatively stable with time to onset decreasing by about 0.2 min/day in controls.