Our interest in the area of m4 muscarinic antagonists had led us to study a series of benzoxazine isoquinolines. One of the most potent and selective compounds of this series is example 1 with an IC50 value of 90.7 nM at m4 receptors, and 72-fold (m1), 38-fold (m2), 10-fold (m3), and 82-fold (m5) more selective compared to the other receptors. The synthesis and receptor binding affinity of analogs of 1 are reported.