The inhibitory binding constants, Ki, at the adenosine A1 receptor in rat brain have been measured at 0 and 25 degrees C for 25 typical ligands. The Ki ratios at the two temperatures are greater and smaller than unity for adenosine agonists and xanthine antagonists, respectively. These results suggest that two-temperature measurements of in-vitro Ki constants represent a simple method of discriminating between in-vivo agonistic and antagonistic behaviour of A1 adenosine receptor ligands.