Relative responding on concurrent schedules: indexing ethanol's reinforcing efficacy.

Five rats were trained to lever press on a concurrent schedule resulting in 3 second access to a dipper (0.1 ml) with either ethanlo (5% v/v) or water. Stable concurrent fixed ratio (FR8 FR8) ethanol and water responding was established. The proportion of responses for ethanol relative to the total number responses varied among rats from 77% to 98%. While the absolute number of responses for ethanol varied greatly day to day, the proportion of ethanol responding was stable. Absolute number of responses for ethanol and proportion of ethanol responding both declined as the body weight of the rats was allowed to increase from initial 80% of ad lib to 110%, with absolute responding showing the greater change. It was concluded that the relative measure of responding on a concurrent schedule could be useful as an index of a drug's reinforcing efficacy, but must be interpreted carefully in regards to the context of drug availability.