Previously, we reported on PD 102807 (41) as being the most selective synthetic M(4) muscarinic antagonist identified to date. Synthesized analogues of 41 showed no improvement in affinity and selectivity at that time. However, several newly synthesized compounds exhibit a 7-fold higher affinity at M(4) receptors and demonstrate a selectivity of at least 100-fold over all other muscarinic receptor subtypes. For example, compound 28 showed an affinity of pK(i) = 9.00 at M(4) receptors and a selectivity of M(1)/M(4) = 13 183-fold, M(2)/M(4) = 339-fold, M(3)/M(4) = 151-fold, and M(5)/M(4) = 11 220-fold. This high selectivity along with high affinity has not been reported for any synthetic muscarinic antagonist, nor for natural occurring M(4) antagonists such as the M(4) selective Eastern Green Mamba venom MT3 (M(4) pK(b) = 8.7, M(1)/M(4) = 40-fold, M(2)/M(4) > or = 500-fold, M(3)/M(4) > or = 500-fold, and M(5)/M(4) > or = 500-fold). Derivative 24, a compound with a high selectivity pattern as well, has been tested for in vivo efficacy. It was able to block the L-3,4-dihydroxyphenylalanine accumulation produced by CI-1017, an M(1)/M(4) selective muscarinic agonist, in the mesolimbic region and striatum, which suggests that 24 is capable of crossing the blood-brain barrier and confirms the pharmacokinetic data obtained on this compound. This is evidence that suggests that agonist-induced increase in catecholamine synthesis observed in these regions is mediated by M(4) receptors.