Rats from the N/Nih heterogeneous stock have been selectively bred for high (HAS) or low (LAS) initial sensitivity to injected ethanol as measured by duration of the loss of the righting reflex. The selection for ethanol sensitivity in these lines apparently has reached a maximum. These lines are useful to elucidate the central nervous system mechanisms of the genetic differences between the lines and also provide clues to the mechanisms of ethanol's action. We have found that: 1) ethanol, etomidate, and ketamine but not propofol produce different sleep times and brain levels of the drug on awakening between these two lines; 2) only ethanol, etomidate, and ketamine produced significant differences between the HAS and LAS rats in GABA-mediated stimulation of chloride uptake into brain microsacs; 3) GABA, propofol, and etomidate decreased the Kd for flunitrazepam binding to whole-brain membranes but equally in both lines. Neither ethanol nor ketamine had an effect; 4) only GABA, ethanol, and etomidate increased the Kd for TBPS binding and only GABA decreased Bmax of TBPS binding. As with the previous selection for ethanol sensitivity in mice (short and long sleep) these lines of rats have very marked line differences in GABA-mediated events, and these are correlated with the sedative effects of ethanol. From these and previous studies we know that the major differences between selected lines of mice and rats are that the mouse lines are not differentially sensitive to halothane or pentobarbital while the rat lines are. However, the mouse lines are differentially sensitive to propofol and the rat lines are not. These data should be useful in dissecting the actions of ethanol at the GABA(A) receptor.