Ihave two guinea pigs and two rabbits that both love spinach! But I
worry about the amount of oxcalates they may be getting since too much
can cause future kidney stones.

Hi flash7

Oxalates do not seem to play a major role in the formation of urinary
tract stones in guinea pigs.
One paper surveying 20 cases shows only three instances of oxalates in
stones and in all of those oxalate was a minor component. (Translation
from the German is a little ordinary but the content is there.
Contains pictures of surgical procedures.)
http://www.wugnet.de/mitglieder/wenzel/urolith.htm

Sufficient water intake (as in humans) seems to be th emajor
contributing factor in stone formation. Intake should be around 100 ml
water/kg of body weight.
The other major factor is calcium intake, and more specifically the
calcium to phosphate ratio. Foods with a high calcium:phosphorus ratio
are preferable meaning leafy foods are prefereable to seeds, roots and
fruit.
http://guinealynx.info/forum_stones.html


For further informtion you might like to join the guinea pig stones mailing list:
http://gpigs-database.org/mailman/listinfo/stones_gpigs-database.org

For general guinea pig health links this is a excellent resource:
http://www.treenspigs.com/care/carelinks.html