The brain levels of meperidine and three N-alkyl homologues were determined at equal analgetic iv doses in mice. The relative levels of the four compounds in brain were found to be closely proportional to their ED50 doses even though the compounds exhibit a wide range in partition coefficient and metabolic N-dealkylation. While lipid solubility and metabolism are undoubtedly important factors in the overall time-course brain levels, it appears that during the period of analgetic measurement (5-60 min after injection) these factors do not profoundly affect the relative brain levels because peak uptake occurs within the first 5 min after administration of the homologues. As a consequence, the observed ED50 potencies appear to provide a fair approximation of the relative receptor affinities of the four homologues. N-Dealkylation was observed as a major metabolic transformation by mouse liver in vivo for all four compounds, and the extent of this N-dealkylation was found to directly correspond to the rates of N-dealkylation by mouse liver homogenate seen in an earlier study.