WaPo:

More and more foreign governments are sponsoring such excursions for lawmakers and their staffs, though an overhaul of ethics rules adopted by Congress five years ago banned them from going on most other types of free trips. This overseas travel is often arranged by lobbyists for foreign governments, though lobbyists were barred from organizing other types of congressional trips out of concern that the trips could be used to buy favor.

The overseas travel is covered by an exemption Congress granted itself for trips deemed to be cultural exchanges.

A Washington Post examination of congressional disclosures revealed the extent of this congressional travel for the first time, finding that Hill staffers had reported taking 803 such trips in the six years ending in 2011. Lawmakers themselves are increasingly participating, disclosing 21 trips in 2011, more than double the figure in prior years.