Using the quantitative microdialysis 'point of no net flux' method, we estimated the interstitial free concentration (IFC) of (+/-)-7-hydroxy-2-(N,N-di-n-propylamino)tetralin (7-OH-DPAT) in the dorsal striatum of freely moving rats after i.p. administration of the drug at the dose of 18.3 mumol/kg. The maximal IFC of 7-OH-DPAT was found to be 1.61 microM 20 min after the injection. Due to the approximately linear relationship between dose and dialysate concentration observed, it may be inferred that the behaviourally active 7-OH-DPAT dose of 0.12 mumol/kg should give an IFC which does not exceed 10 nM. It is concluded that in vivo effects observed following 7-OH-DPAT i.p. administration at doses lower than 0.12 mumol/kg might be considered as mediated by the dopamine D3 receptor.