Synaptic plasticity is generally believed to provide a cellular mechanism for learning and memory. One manifestation of synaptic plasticity, long-term potentiation in the CA1 region, was compared in hippocampal slices from young and older rats, both control animals and streptozotocin-treated diabetics with moderate hyperglycemia ( approximately 15 mM). Long-term potentiation of excitatory synaptic potentials, elicited by tetanic stimulation or by 2-deoxy-D-glucose application, was readily obtained in slices from young (four-month-old) control and diabetic rats, but not in slices from middle-aged (12-month-old) control rats. Both forms of potentiation, however, could be elicited in slices from 12-month-old diabetics. The unexpected finding that long-term potentiation is restored in moderately hyperglycemic older rats suggests that the blood glucose level of older animals may be important for synaptic plasticity and perhaps for the ability to learn.