In rats and mice, the genetically mediated extent of the mossy fiber projection that synapses on the basal dendrites of hippocampal pyramidal cells is inversely correlated with rate of two-way avoidance (shuttle-box) learning. Postnatal hyperthyroidism, induced in 51 rat pups, resulted in marked variations of this infrapyramidal mossy fiber projection. The number of trials required for criterion performance of these rats in adulthood remained correlated with the neuroanatomical trait (r = 0.74, P less than 0.0001).