This is a common complaint in male guinea-pigs (boars) and is caused by anal "impaction". When you pick him up, rest your hand gently under his rump so that his testicles are against your skin. In a normal boar, this should feel soft. The pouch is normally coated in a white powdery substance which would smell "foul" if you made an effort to smell it. However, when impacted, this smell would be more pungent, and would be obvious in the vicinity of your boar. The appearance would be darker and you might be able to spot pieces of feces, hair and even shavings.

You can take a cotton bud soaked in warm water and very gently moisten the lump in a circular motion. Do not pull at the lump as this may hurt the boar. You can also use mineral oil instead of warm water which may work better in severe cases. The lump should gradually loosen and fall out, after which you should wash him with small warm, mildly soapy, water.

To prevent recurrence, you should check your guinea-pig once a week, particularly if you start smelling him again. It is more likely to recur in older boars, where the muscles surrounding the anal cavity are weaker. If there are any signs of impaction starting again, gently cleanse with a cotton bud soaked in warm water.