The behavioural effects of two antidepressants with opposite molecular mechanisms, tianeptine 7-[(3-chloro-6,11-dihydro-6-methyldibenzo[c,f][1,2]thiazepin - 11-yl)amino]heptanoic acid S,S-dioxide, CAS 66981-73-5) 5 mg/kg p.o., a serotonin reuptake enhancer, and fluoxetine (+/-)-N-methyl-3-phenyl-3-[(alpha, alpha, alpha-trifluoro-p- tolyl)oxy]propylamine, CAS 54910-89-3) 5 mg/kg p.o., a serotonin reuptake inhibitor, were compared after single and prolonged administration (7 and 14 days) once daily). In all experiments the drug effects were noted at the peak activity time: 30 min after tianeptine and 60 min after fluoxetine administration. In the immobility time test both drugs had a shortening effect on immobility time only after prolonged administration or, in single treatment, after joint administration. A different pattern was observed in the two compartment test: both antidepressants showed anxiolytic effects after single and prolonged treatment. However, when the drugs were given in joint administration, the anxiolytic effects were entirely abolished after single as well as prolonged treatment. In reference spatial memory test (food finding time in the maze) tianeptine had no effect, whereas fluoxetine caused, after single and prolonged treatment, a very marked improvement of reference memory. Joint administration of both drugs resulted in worsening the effects on memory in comparison to fluoxetine alone, but the results were still significantly better vs. control. In the test for sedative action (in the Activity Meter AM-1, where the movements of the animals are counted electronically) only after prolonged treatment with tianeptine a diminished locomotor activity could be observed. It is concluded that in the action of the drugs (beside the effect on serotonin uptake) other mechanisms must play an important role. The diminished locomotor activity after tianeptine suggests an influence on the dopaminergic or GABA-Receptor system.