The anticholinergic antiparkinsonian drugs biperiden, benztropine, trihexyphenidyl, methixene, and procyclidine were compared with atropine and pirenzepine, as well as with orphenadrine, amantadine and some standard antidepressives and neuroleptics in their ability to inhibit the binding of tritiated quinuclidinyl benzilate (QNB) to the muscarinic receptors in rat brain cortical tissue. Most of the antiparkinsonian drugs studied were potent inhibitors of (-)3H-QNB binding, when compared to atropine (IC50-value = 0.22 microM), the IC50-values ranging from 0.0084 microM (biperiden) to 0.07 microM (procyclidine). Orphenadrine had a low and amantadine no evident affinity for muscarinic receptors. With the exception of pirenzepine and biperiden the inhibition curves were steep and parallel, giving linear Hill plots with coefficients close to unity. The binding profile of atropine, pirenzepine, and biperiden was further studied in heart and lung tissues, atropine showing only small divergences in its binding to the different tissues, but biperiden and pirenzepine having five to ten times lower affinity in the peripheral tissues than in the brain. The results confirm the high affinity of most of the antiparkinsonian drugs for brain muscarinic receptors. The dissociation constants agree with the average clinical doses of the drugs. It must be remembered, however, that the binding data may represent multiple events at receptor sites because most of the drugs used are mixtures of stereoisomers. Thus further studies using individual enantiomers are needed to compare more directly binding data between the compounds.