Quantitative receptor autoradiography was used to assess GABAB receptor binding in three primary subdivisions of the inferior colliculus (IC): dorsal cortex (DCIC), external cortex (ECIC), and the central nucleus (CIC) of 3-, 18-20-, and 26-month-old Fischer 344 rats. GABAB binding sites were localized using [3H]GABA in the presence of a saturating concentration of isoguvacine, a selective GABAA receptor agonist, to displace [3H]GABA bound to GABAA receptor sites. In the three IC subdivisions examined, GABAB receptor binding was significantly reduced in 26-month-old rats when compared to 3-month-old rats (DCIC, -44%; ECIC, -36%; CIC, -32%; p < 0.05). For comparison, GABAB binding was determined in the portion of cerebellum located in the recess of the IC. In the molecular layer of this region, there was no statistically significant differences in receptor binding between 3, 18-20-, and 26-month-old rats. In addition, there was not a significant age-related change in the cross-sectional area of the IC. These findings provide additional evidence to support the existence of selective age-related changes in GABA neurotransmitter function in the rat IC.