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Injection of pentobarbital after a rat has consumed saccharin solution usually produces a mild aversion to the saccharin. However, the pentobarbital-produced aversion is eliminated or attenuated by prior pairings of pentobarbital injections with lithium injections. This is called the Avfail (aversion failure) effect. The present experiments dealt with the effect of the temporal relation of the pentobarbital injection to the lithium injection. After forward pairings (pentobarbital before lithium) with delays between the two injections varying among groups from 2.5 min to 320 min, Avfail was invariably obtained. There was little effect of the length of the forward delay, although the Avfail effect appeared to be slightly stronger at 30-40 min or so. When the two drugs were injected simultaneously or in a backward sequence, there was a weakening of the flavor aversion produced by pentobarbital, but this is attributable to habituation to the drugs, not to Avfail.Associative learning occurs when two drug states occur in sequence. It has been demonstrated by injecting rats with a sedative dose of pentobarbital 30 min prior to injection of a toxic dose of lithium. After four to eight such pairings, pentobarbital loses its capacity to produce a saccharin aversion; that is, there is no later saccharin aversion if pentobarbital is injected into the rat just after it has consumed saccharin solution. Control rats that receive equal experience with pentobarbital and/or lithium do not exhibit this effect (Revusky, Taukulis, & Coombes, 1980;Revusky, Taukulis, Parker, & Coombes, 1979). Hence the effect is not due to habituation. The involvement of conditioning is further indicated by findings that a similar effect is obtained with drugs other than pentobarbital and lithium. However, it is not obtained with all combinations of drugs (Cunningham & Linakis, 1980; Revusky, Coombes, & Pohl, in press).Although this effect is due to conditioning,