The biodistribution of [3H]Ro 15-1788 in control and stress-loaded mice (forced swimming) was compared. In control mice, carrier-free [3H]Ro 15-1788 was selectively and highly distributed in Bz receptor rich brain regions, while radioactivity in the brain was very low following administration of carrier-added tracer, which suggested that in vivo non-specific binding of this tracer was very low. Significant changes in biodistribution of carrier-free [3H]Ro 15-1788 were observed in stress-loaded mice, which strongly indicated that in vivo binding availability of Bz receptor in the brain was rapidly and reversibly reduced by acute stress. The degree of these changes was very dependent upon the stressful conditions, such as swimming duration and water temperature, and a significant alteration in biodistribution of [3H]Ro 15-1788 was particularly observed in the cerebral cortex. Simplified Scatchard analysis of in vivo binding of this tracer was performed, and results suggested that these alterations were mainly caused by changes in the Kd value rather than the Bmax value.