1. The analgesic efficacy of 1-(m-methoxyphenyl)-2-(dimethylaminomethyl)-cyclohexan-1-ol (tramadol; Tramal) (75 and 100 mg) was investigated in 22 young, healthy volunteers by means of an algo-pupillometric method. 2. Tramadal itself causes a slight miosis which becomes statistically significant only 3 h after administration. 3. In the algo-pupillogram the effect of tramadol is shown to be biphasic during the first 80 min. Thereafter the pupillary reaction steadily decreases, reaching a minimum after about 3.5--4 h in both dosages. 4. At the time of maximum effect the pupillary reaction of subjects given 75 or 150 mg tramadol is calculated to be equal to that of untreated individuals to whom only 60% or 10%, respectively, of the stimulus intensity has been applied.