This study was performed in order to investigate the extent and severity of cyproheptadine effects on psychomotor performance, mood and memory functions and to compare them to the effects of DL carnitine, another appetite stimulant. Twelve healthy volunteers received 2 doses (at 800 am and 1200 am) of 6 mg cyproheptadine, 1600 mg DL carnitine and placebo on separate days at a weekly intervals. The study followed a double-blind, latin-square design. Assessment of dependent variables was performed 1 h after the first and 1 h and 5 h after the second administration of the drug. On each of these occasions, the following measurements were performed: choice reaction time (CRT), critical flicker fusion (CFF), digit symbol substitution test (DSST), short-term memory (paired words association test), long-term memory (picture test) and 100 mm visual analogue scales of subjective ratings (VAS). Cyproheptadine significantly impaired objective measures (CFF) and subjective ratings both at 1 h and 5 h after the second dosage. Compared with cyproheptadine, DL carnitine induced a slight improvement in psychomotor performance as assessed by CRT. None of the drugs had any effect on memory and on appetite at the doses studied. In conclusion, cyproheptadine at usual doses had a sedative effect, the intensity and duration of which implied a certain risk in performing daytime functions eg when driving, or manipulating machines. DL carnitine had no effect on vigilance.