GAP-43 is expressed in axonal growth cones during development and regeneration. High levels of GAP-43 persist in the adult hippocampus, reflecting perhaps the ongoing neurogenesis and synaptic plasticity characteristic of this brain region. Since the morphology and survival of hippocampal neurons are altered by adrenal steroids and aging, we have investigated the effects of adrenalectomy and of aging on the expression of hippocampal GAP-43 mRNA, as a molecular marker for these events. Adrenalectomy for 7 days results in an increase in GAP-43 mRNA expression, as assessed by in situ hybridization, in the pyramidal cell layers of the hippocampus. Both male and female aged rats exhibit levels of GAP-43 mRNA in hippocampal pyramidal cells lower than those of their younger counterparts.