This study concerns effects of the testes on two macromolecules in the rat hippocampus that were previously not known to be responsive to this endocrine axis. Castration for 3 weeks elevated the expression of glial fibrillary acidic protein (GFAP) and sulfated glycoprotein-2 (SGP-2) in male rat hippocampus, as shown by Northern blots and immunocytochemistry. SGP-2 mRNA was colocalized with GFAP, implying increased prevalence in astrocytes after castration. During hippocampal responses to deafferentation by entorhinal cortex lesions that damage the perforant path and induce synaptic reorganization, both mRNA and protein for SGP-2 and GFAP increase. Moreover, prior castration had an additive effect with entorhinal cortex lesions in the increase in GFAP and SGP-2 mRNA. These data suggest that testicular hormones regulate hippocampal astrocyte activity in intact adult rats as well as during synaptic reorganization in response to deafferenting lesions.