> ...rabbits can be easily trained to use a litter tray, sometimes with more reliability than your average cat!
 
> The natural instinct of a wild rabbit to use one area as its latrine is still apparent in its domestic counterparts.  (1)


The actual process is very similar to pad training a dog or litter box training a cat. Keep the rabbit confined to a small area while training, move any "accidents" to the litter box, and the rabbit will naturally start using that area for its business. The source link has the details.


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(1) [Litter Training Your Rabbit][2]
Emma Magnus MSc  
Association of Pet Behaviour Counsellors  
apbc.org.uk


  [1]: http://www.apbc.org.uk/articles/rabbitlitter
  [2]: http://www.apbc.org.uk/articles/rabbitlitter