The giant interneurons (GIN) from the cockroach CNS undergo two major physiological changes during the postembryonic developmental period: (A) a marked decrease in the number of afferent pathways innervating the GIN at the metathoracic ganglion (Ts); and (B) a gradual decrease in the safety factor for impulse propagation along the intraganglionic segment in T3. In 100% of the experiments (n greater than 100) performed on GIN from early developmental stages, spontaneous postsynaptic potentials (SPSPs) were recorded; in adults, on the other hand. SPSPs have been recorded in only 34% of the experiments (n = 74). Evoked synaptic potentials can be elicited in nymphal stages by stimulation of 8 nerves of T3, the contralateral connectives, ipsi- and contralateral nerve roots 2, 3, 5, and by stimulation of adjacent GINs. In adult, PSPs can be evoked by stimulation of adjacent GINs, and contralateral thoracic connectives, but not from nerves 2, 3 and 5. The functional disappearance of synaptic inputs to the GINs does not reflect a general phenomenon of reduction in synaptic transmission efficacy. In previous studies it was demonstrated that high frequency stimulation of adult GIN leads to blockage of impulse propagation in T3. In nymphal stages, the safety factor for propagation of impulses along T3 is higher. The reduction in safety factor appears gradually during the postembryonic developmental period. From analysis of the mechanisms underlying the elimination of functional afferent pathways and the appearance of low safety factor (see consecutive paper by Yarom and Spira) it is concluded that the functional elimination of afferents is a consequence of decreased transmission efficacy, while the appearance of low safety regions for impulse propagation is a consequence of morphological changes of the GIN segment within ganglion T3.