Pressure treated wood is toxic. For protection of humans and animals this wood should be sealed or boxed off.
If your barn will be sealed and the posts are exposed inside they should be painted with a shellac or varnish prior to enclosing them in the wall.
For horse barns they can be roughed out with 1 x 8 boards to box the posts to a height of eight feet or so. This will prevent horses and livestock from rubbing and chewing on them.
Treatment chemicals and methods have advanced to the point that "Green" non-toxic foundation posts are available that are ready to go for horse stalls, internal posts, and for those with general conservation concerns.
Barn Roof Styles
Why Pole Barns are Better