The 'disector' method was used to estimate number and size of simple non-perforated and complex 'perforated' synapses and their 'perforations' in the hippocampal CA3 area of 3, 12, 24 and 30 months old rats. A decrease with age from 3 to 24 months of age in the number of non-perforated synapses per neuron was found. Both non-perforated and perforated synapses increase in size with age. The perforations in the perforated synapses increase in size and number from 3 to 24 months. In all 4 age-groups, perforated synapses are larger and more complex (perforations) than non-perforated synapses. The increase in size (and complexity) of the synapses with age was considered as increased efficacy of the synapses. It was suggested that this enhanced efficacy of synaptic transmission may reflect a compensatory reaction of synapses proper in response to the observed loss of synapses. The latter, in itself, may lead to impairment of the synaptic circuitry.