Three different methods for the culture of rumen microorganisms (Hungate's technique, the Hohenheim in vitro gas production method, and the semicontinuous rumen simulation technique) were employed to study the influence of various alkaloids (sparteine, lupanine, cytisine, atropine, quinidine, lobeline, harmaline, arecoline, nicotine, caffeine, pilocarpine, gramine, senecionine, and monocrotaline) on rumen microorganisms. Rumen micro-organisms from naive ruminants (sheep, cattle) that had not been exposed to the alkaloids before were generally not able to degrade most of the alkaloids. Only the alkaloids pilocarpine, gramine, and monocrotaline appeared to be degradable. Rumen microorganisms from a sheep preconditioned to lupin alkaloids tolerated lupanine much better than nonadapted microorganisms, but no degradation occurred. The findings indicate that the main site of detoxification in naive ruminants is not the rumen but more likely the liver and kidneys as in nonruminants.