Monbaliu, J., Degryse, A.‐D., Ooms, L.A.A., Van Dijk, P., Lagerweij, E., Michiels, M., Woestenborghs, R.&Heykants, J. Hypnoanalgesia with R 8110/ fentanyl in the dog: pharmacodynamic and pharmacokinetic interactions.J. vet. Pharmacol. Therap.11, 63–70.The pharmacokinetics and clinical effects of the short‐acting hypnotic R 8110 and of the narcotic analgesic fentanyl were studied in the dog. The effects of separate intravenous (i.v.) injections of R 8110 (4 mg/kg) and fentanyl (0.015 mg/kg) and of concurrent i.v. injection of the two were studied. After administration of R 8110, induction of hypnosis occurred within 1 min, maximal depth of anaesthesia and satisfactory analgesia and muscle relaxation were obtained after 5 min. The effects had decreased within 15 min and full recovery occurred within 30 min. Fentanyl alone produced neither hypnosis nor muscle relaxation. When fentanyl and R 8110 were given simultaneously, the duration of hypnosis was doubled in comparison with R 8110 alone. Moreover, markedly improved and longer lasting analgesia and muscle relaxation were observed with the combination. When the drugs were injected together, the plasma concentrations of R 8110 were initially much higher than after separate injection of R 8110, but they became similar after 30 min. Although statistically non‐significant, fentanyl reduced the total plasma clearance of R 8110 (31.1 ± 6.9 MS. 21.9 ± 2.3 ml/kg/min) and decreased the volume of distribution (3.78 ± 1.83vs.2.23 ± 0.90 1/kg,P<0.05). Fentanyl did not alter the elimination half‐life ofR8110. R8110 had no apparent influence on the pharmacokinetic