Tetrahydropyrido[3,4-d]pyrimidines and related analogues are provided, of the formula: in which variables are as described herein, as are methods for their preparation and use. Such compounds may generally be used to modulate ligand binding to histamine H3 receptors in vivo or in vitro, and are partially useful in the treatment of a variety of disorders in humans, domesticated companion animals and livestock animals. Pharmaceutical compositions and therapeutic methods are provided, as are methods for using such ligands for detecting histamine H3 receptors (e.g., receptor localization studies).