The selectivity of N,N-dipropyl-2-[4-methoxy-3-(2-phenylethoxy)phenyl]-ethylamine monohydrochloride (NE-100) for sigma 1 and sigma 2 binding sites was studied by means of binding of [3H](+)-pentazocine and [3H]1,3-di-o-tolylguanidine ([3H]DTG) in guinea pig brain. NE-100 inhibited [3H](+)-pentazocine binding to sigma 1 binding sites potently with an IC50 value of 1.54 +/- 0.26 nM while it had a weak effect on [3H]DTG binding to sigma 2 binding sites. The inhibitory effect of NE-100 on [3H](+)-pentazocine was 55 times more potent than that on [3H]DTG binding. These results suggest that NE-100 is a potent and selective ligand for sigma 1 binding sites.