The effects of hypoxia were studied in hippocampal slices prepared from animals of 10-120 days of age. Excitatory synaptic responses could be evoked at all ages, while inhibitory responses began to appear 14-17 days postpartum; both types of responses became maximal at approximately 30 days of age. Inhibitory responses were somewhat more susceptible to the deleterious effects of hypoxia than were excitatory ones. Very young animals showed virtually no effects of hypoxia. However, past 30 days of age, all types of synaptic response became progressively more sensitive to the effects of hypoxia.