1. The affinities of urapidil derivatives and other antagonists for alpha 1-adrenoceptors labelled by [3H]-prazosin were determined on membranes of six different rat tissues. 2. Urapidil and its 5-acetyl-, 5-formyl- and 5-methyl-derivative displaced [3H]-prazosin from alpha 1-adrenoceptor binding sites in a concentration-dependent manner which varied with tissue. IC50 values were lower in vas deferens, hippocampus and cerebral cortex than in heart, liver and spleen. For 5-methyl-urapidil, binding to two distinct sites could be demonstrated with mean K1 values of about 0.6 and 45 nM. Saturation binding studies with [3H]-prazosin in the presence of 5-methyl-urapidil indicated a competitive type of interaction between 5-methyl-urapidil and [3H]-prazosin. 3. The proportion of [3H]-prazosin binding sites with high affinity for 5-methyl-urapidil was 58% in vas deferens, 69% in hippocampus, 41% in cerebral cortex and 23% in myocardium. In liver and spleen virtually no high affinity sites were found. These values were in good agreement with the percentages of binding sites with high affinities for WB-4101 and phentolamine, indicating that all these antagonists bind to the same subtype of alpha 1-recognition sites, whereas other alpha-antagonists like BE 2254, yohimbine and unlabelled prazosin did not discriminate between two binding sites. 4. Preincubating membranes of the cerebral cortex with chloroethylclonidine preferentially inactivated [3H]-prazosin binding sites with low affinity for 5-methyl-urapidil. 5. The antagonist potencies of 5-methyl-urapidil and WB-4101 against alpha 1- adrenoceptor-mediated contractile responses were higher in vas deferens than in myocardium. The alpha 1-mediated effects in vas deferens but not in the heart were highly susceptible to nitrendipine. 6. Using 5-methyl-urapidil, the existence of two distinct alpha 1-adrenoceptor recognition sites could be demonstrated which correspond to the proposed alpha 1A- and alpha 1B-subtypes. Since 5-methyl-urapidil is one of the ligands with most selectivity between these subtypes in binding studies it may serve as a valuable tool for such investigations.