1. Actions and interactions on performance and respiration of single intramuscular doses of 0.15 mg kg-1 nalbuphine and oral haloperidol twice daily for 2 days were studied double-blind and cross-over in 12 healthy volunteers. 2. Objective measurements of performance (choice reaction, tracking, attention, flicker fusion, Maddox wing, digit symbol substitution) and respiratory function (minute volume, end-tidal carbon dioxide), and subjective assessments on visual analogue scales were done at baseline and 1 h, 2.5 h and 4 h after the injection of nalbuphine. Plasma concentrations of nalbuphine were estimated by radioreceptor ([3H]-dihydromorphine) assay, and those of prolactin and growth hormone by radioimmunoassay. 3. Nalbuphine affected digit substitution, reaction time, extraocular muscle balance and flicker recognition, and depressed respiration most clearly 1 and 2.5 h post injection. Motor skills were impaired only briefly. Haloperidol alone proved inert on performance but enhanced the decremental effects of nalbuphine on digit substitution and exophoria at 1 h. It did not interact with nalbuphine on the ventilatory function. 4. Plasma concentrations of nalbuphine expressed as morphine equivalents ranged from 5 to 52 ng ml-1, indicating considerable mu-opiate affinity. Treatment with haloperidol increased plasma prolactin moderately whilst nalbuphine raised it markedly 1 and 2.5 h post injection. Nalbuphine elevated plasma growth hormone at 1 h post injection only.