1. Buprenorphine given intravenously (6 micrograms kg-1) was examined for its antinociceptive activity in unrestrained sheep using devices to measure thermal and mechanical thresholds. 2. The plasma levels of buprenorphine following intravenous injection over the time period of the antinociceptive testing were measured using a radioimmunoassay. 3. Buprenorphine produced a clear antinociceptive effect lasting for up to three and a half hours when measured by the thermal threshold test, but no detectable antinociception in the mechanical test. 4. The plasma levels of buprenorphine indicated that the drug was rapidly distributed in a manner not dissimilar to that reported in man, although individual animals showed a wide variation in some parameters. 5. When plasma levels of the drug were high (less than 700 pg ml-1) during the first sixty minutes, no antinociceptive activity in the thermal test could be detected, which may be due to the slow receptor kinetics shown by this drug.