The effects of administration of antipsychotic drugs (haloperidol, loxapine, sulpiride; 1-32 day time course) on the rat brain mRNA levels of the dopamine D5 receptor has been assessed using solution hybridisation with oligonucleotides. In contrast with the previously reported increases of D1, D2 and D3 receptor mRNA levels in identical experiments, no changes were found in dopamine D5 receptor mRNA levels, suggesting that the mechanism of regulation of D5 receptor mRNA is different to the other cloned dopamine receptors. We also conclude that up-regulation of the D5 receptor is not likely to be involved in the mechanism of action of antipsychotic drugs.