Morphogenesis of the hippocampal mossy fiber synapses (MFSs) in fetal and postnatal rhesus monkeys was examined by electron microscopy. Typical invaginated MFSs were absent on gestation days 90 and 109 and were detected infrequently on day 132. In older fetuses and postnatal animals of 16 days and 9 months of age, the percentage of invaginated MFSs increased and the synapses exhibited increases in the number of invaginated spines and density of synaptic vesicles and greater maturity of the spines. At all ages, MFSs exhibited a wide range of maturity which may be correlated with the reported protracted generation of dentate granule cells in the rhesus monkey. The pattern of morphogenesis of MFSs in the monkey suggests that as in the rat, it is very likely that the mossy fibers contact pyramidal cells prior to the emergence of the 'thorny excrescences' and that this is followed by progressive maturation of the MFSs. The time-course of major morphogenetic modifications in MFSs in the rhesus monkey corresponds to the period of elaboration of complex spines and synapses in the dentate molecular layer. On this basis, it is suggested that these two processes are interrelated developmental events.