The effect of dopaminergic D1 receptor blockade on the expression of long-term potentiation (LTP) was investigated in the rat hippocampal CA1 region in vitro by extracellular recordings (by measuring the population spike amplitude and the field EPSP). The presence of the very selective D1 receptor blocker SCH 23390 at a concentration of 0.1 microM during tetanization with 3 trains of 100 impulses (100 Hz) resulted in a prevention of late LTP stages (greater than 1-2 h). When SCH 23390 was added to the bath medium immediately after tetanization, an influence on established LTP could not be observed during the first 3 h investigated.