Purpose. To investigate the blood-brain barrier (BBB) passage of theM1 muscarine agonist Lu 25-109(5-(2-Ethyl-2H-tetrazol-5-yl)-1,2,3,6-tetrahydro-methylpyridine) and potential metabolites using in vivomicrodialysis. Methods. Anesthetized rats were administered an intravenous infusionof one of seven analogs with a Log D7.4 ranging from 0.35 to −2.4.Microdialysis probes were implanted in the brain and the jugular vein.The integrity of the BBB was evaluated using2-amino-3-(3-hydroxy-5-phenylisoxazol-4-yl)propionic acid (APPA), a compound notexpected to penetrate the BBB. The data was corrected for in vitrorecovery. Results. Lu 25-109, Lu 24-165 (demethylated Lu 25-109) and Lu25-077 (N-demethylated Lu 25-109) entered the brain in a 1:1 ratio withthe blood. Although Lu 29-081 (hydroxylated Lu 25-109) presented asimilar Log D7.4 to Lu 25-109 and Lu 24-164, it entered the brain witha lower brain:blood ratio of 0.5. Lu 32-181 (Lu 25-109 N-oxide), Lu35-026 (deethylated and oxidized Lu 25-109) and Lu 31-126(deethylated Lu 25-109) were not detected in the brain samples, indicating nopenetration. Infusion of Lu 25-109 resulted in a time perspective ofthe formation and distribution of the two metabolites Lu 25-077 andLu 32-181. Although the hydroxylated compound (Lu 29-081) had aLog D7.4 of −0.6, within the range 0.35 to −0.83 of the compoundspenetrating the BBB, it showed a brain: blood ratio of 0.5. Lu 35-026showed an unusual infusion profile with a tmax of 100–150 min and asubsequent decrease in blood concentration. Conclusions. Compounds with Log D7.4 above −0.83 penetrated theBBB, whereas compounds below −1.5 did not. Knowledge of LogD7.4 values is not sufficient to evaluate BBB passage because the valuedoes not predict the influence of active transport processes.