The effects on memory and learning of fluvoxamine 50 mg twice a day were compared with those of mianserin 20 mg twice a day and placebo, each given for 8 days in a double-blind cross-over design to nine healthy human volunteers. At least 1 week was left between the 8-day courses of drugs. Subjects were given a learning task (three trial recall of categorisable word lists) before and 3.5 h after a morning dose on day 1 and before their morning dose on day 8. Delayed recall was assessed on days 1, 4 and 8. Fluvoxamine had no effect on memory performance. Mianserin reduced learning and recall after a single dose but had no effect on day 8 of treatment. The single dose of mianserin had no retrograde effect on memory, affected primacy and middle position items but not recency in the serial position curve, and was seen in reduced inter-trial subjective organisation of recall. Subjects' performance on the first trial of the memory task correlated significantly with their performance on a simple reaction time task, with finger tapping speeds and with their subjective ratings of alertness. It was concluded that the impairments of memory produced by one dose of mianserin are partially by-products of the sedative effects of the drug. Tolerance to both memory impairments and sedative effects built up over the 8-day treatment of mianserin.