5,11-Dihydro-11-[(4-methyl-1-piperazinyl) acetyl]-6H-pyrido[2,3-b][-1,4]-benzodiazepin-6-one dihydrochloride (pirenzepine, Gastrozepin) and some tricyclic antidepressant drugs which show a very close relationship concerning the chemical structure were investigated in numerous binding, uptake and enzymatic studies in vitro. With pirenzepine a high affinity binding could be demonstrated only to muscarinic receptors (Ki = 58 nmol/l). In all other studies pirenzepine had a very weak or no effect at all. In contrast, tricyclic antidepressants bound with high but different affinities to various receptors as known from numerous publications. The highest affinities were found with imipramine at the specific imipramine binding sites (Ki = 9.8 nmol/l) and at the alpha 1-receptor (Ki = 39 nmol/l), with desipramine at the muscarinic receptors (Ki = 88 nmol/l), with mianserin at the H1-(Ki = 3.4 nmol/l) and 5HT2-receptors (Ki = 7.3 nmol/l). Moreover, imipramine and desipramine showed their known substantial inhibition of noradrenaline and/or 5-hydroxytryptamine uptake. Thus, a homogeneous affinity or activity profile of the antidepressants studied does not exist. The only common property of pirenzepine and the tricyclic antidepressants was found to be the high affinity binding to the muscarinic receptors which might explain the common antisecretory action of these agents. Because of the unique specificity of pirenzepine lacking all other effects of the tricyclic antidepressants as demonstrated in this study, it is very unlikely that this drug exerts any antidepressant-like central action.