Atenolol and metoprolol are beta 1-selective adrenergic receptor blockers, devoid of local anesthetic and intrinsic sympathomimetic properties. Their antihypertensive and hypotensive activities are equivalent. They differ with respect to their lipophilic character, as is apparent from their octanol/buffer (pH, 7.4 at 37 degrees C) partition coefficients: metoprolol, 1.084; atenolol, 0.012. We compared the two agents with regard to their acute hemodynamic effects and degree of penetration into the cerebrospinal fluid (CSF) and into the brain. For this purpose [3H]-metoprolol and [14C]-atenolol were injected either intravenously or into the left vertebral artery of chloralose-anesthetized cats. With both routes of administration, metoprolol, the more lipophilic of the two compounds, achieved much higher concentrations in the CSF and in the pontomedullary region than did atenolol. One hour after completion of an intravenous injection, the concentration of metoprolol in the CSF was about 6.5 times higher than that of atenolol; after administration into the vertebral artery, the difference was about ninefold. In spite of these considerable differences, the hypotensive and bradycardic activities of both drugs, administered intravenously or into the left vertebral artery, were virtually the same. These results suggest that the acute hemodynamic effects of metoprolol and atenolol are probably not due to an action within the central nervous system, but rather to a peripheral mechanism.