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The development of synaptic junctions in rat occipital cortex has been studied at 7 and 20 days postnatal in control and undernourished animals. In addition, adult control cortex has been compared with cortex derived from animals undernourished until 35 days postnatal and then nutritionally rehabilitated. Emphasis has been placed on material stained with phosphotungstic acid (E-PTA technique) although osmicated material has been examined. The body weights of the undernourished animals were lower than those of their age-matched controls, the deficit at 20 days being 55%. The number of synaptic junctions per unit area of the molecular layer was lower in the undernourished animals at 7 and 20 days, the most prominent deficit being at 20 days. In an attempt to detect ultrastructural correlates of undernutrition, E-PTA stained synaptic junctions were divided into 5 types (A-E), based upon their presynaptic characteristics are reflecting varying degrees of maturity. During normal development the immature synaptic types, particularly type E, are predominant, giving way to the more mature types (A-C) as development proceeds. Undernutrition however, appears to disrupt this developmental sequence, with a higher than normal percentage of immature synaptic types present at the age studied. Nutritional rehabilitation rectifies the imbalance to a degree, the adult rehabilitated pattern resembling the 20-day control pattern. Preliminary examination of the 20-day osmicated material confirmed the deficit of synapses per area of tissue in the undernourished cortex. It also revealed decreases in the number of vesicles per terminal unit area and in the internal area of the synaptic vesicles in the undernourished tissue.