1. In Lewis (LEW), Fischer 344 (F344), Spontaneously hypertensive (SHR) and Wistar Kyoto (WKY) rats, pentobarbital (PB)-induced sleep time was much longer in female than in male rats. 2. At the time of awakening, brain levels of PB were significantly higher in the female F344 than in the male rats, but there was no sex differences in other strains. 3. Each strain of rats was treated with PB-admixed food for 47 days. There were significant sex differences in mean drug intake of the SHR and LEW strains, but not the WKY and F344 strains during the final concentration. 4. Only female rats exhibited moderate to severe motor impairment by PB. 5. After PB treatment ended, various signs of PB withdrawal occurred in female, but not male, rats. These marked sex differences were observed in all four inbred strains. 6. The sex differences in physical dependence on PB may be due mainly to differences in rates of drug metabolism for the LEW, SHR and WKY rats, and to differences in CNS sensitivity for the F344 rats.