The acute effects of experimentally elevated triiodothyronine (T3) serum levels were investigated in 14 healthy male subjects. After oral application of 100 µg triiodothyronine on 3 consecutive days a battery of neuropsychological tests and rating scales for mood and bodily complaints were administered. Results show slight mood disturbances but no cognitive impairment caused by T3 level changes. Time intervals were estimated as being longer than in euthyroid state, and word production showed a trend to accelerate. It is concluded that the immediate effects of elevated circulating T3 on cognition and mood are merely discrete. More severe cognitive impairments reported in hyperthyroid patients are probably due to long-term effects on the brain.