1. Dissociation constants and Hill coefficients were determined for 13 ligands inhibiting the binding of [3H]flunitrazepam to the benzodiazepine receptors of guinea pig hippocampus at 2 degrees C and 37 degrees C. 2. The ratio of I50 in the absence of gamma-aminobutyric acid (GABA) to that in the presence of 10(-5) M GABA (the GABA-shift) was found to vary from 0.4 to 2.5. 3. There was no correlation between the pharmacological activity of the ligand and the GABA-shift, or between the pharmacological activity and the magnitude of the increase in affinity of the ligand for the receptor as the temperature decreased from 37 degrees C to 2 degrees C. 4. A correlation was observed between the temperature-induced affinity change and the GABA-shift at 37 degrees C.