Yapp, 23, and Brew, 24, fit neatly into the Wales management team's plans to tour South America with a young squad set to benefit from the experience.
"We are looking at players who have the potential to compete for a place in the World Cup squad in 18 months time and both John and Nathan fit that bill," said Wales attack coach Nigel Davies. "Nathan has been in outstanding form for the Dragons in the latter part of this season, after returning from injury, and we have had a good close look at him.
"He is a big, physical lad, an evasive runner and an intelligent rugby player and we are looking forward to working with him.
"He is primarily selected as an outside centre or wing as we've concluded we have adequate cover for the inside centre position vacated by Gavin (Henson) elsewhere in the squad."
Henson withdrew from the Wales tour squad last week and was added to the list of players who will concentrate on pre-season training during the summer, but he was joined on the absentee list yesterday (Thurs) by British Lions prop Jenkins who is undergoing treatment for a shoulder problem.
Jenkins, whose shoulder was operated on after last year's Lions tour of New Zealand, received an injection this week to help settle inflammation and has consequently withdrawn from the tour to be replaced by his front row team-mate.
"John is also a very competitive player who has shown glimpses of what he is capable of for his region this year," added Davies. "He is a prop of great size and stature which will be a boost to us in Argentina and we are hoping his international career can continue where it left off, with an impressive run of wins."
Club/Region: Cardiff Blues
Weight: 19st 3lbs
7 caps (W 7) 2005 E(rep),It(rep),F(rep),S(rep),I(rep),C(rep),Fiji
John Yapp made his Wales debut as a replacement in the 11-9 victory over England in February, 2005 - and came on as an impact player in all five Grand Slam matches. He made his first start for Wales in the match against Fiji. A strong scrummager, he is also a prominent ball carrier and has established himself as a regular in the Cardiff Blues regional side.
Club/Region: Newport Gwent Dragons
Weight: 14st 7lb
International record: 1 cap (W 1; 5 pts - 1T) 2003 R
Nathan Brew, who made a try scoring debut in the 2003 match against Romania at Wrexham, was capped by Wales Senior Schools and Wales U19. In 2002 he made both his Newport senior and Wales U21 debuts, as a replacement against England at Newbury. Scored two tries against England in the IRB U21 World Championship and has represented Wales at Sevens. His younger brother Aled has already played for Wales at U16, U18 and U21 levels.
Wales depart for Argentina on Thursday 1 June 2006 and return Mon 19 Jun
1st Test: Argentina v Wales, Raul Conti Stadium, Puerto Madryn, Sun 11 Jun 2006, 2.40pm (local time)
2nd Test: Argentina v Wales, Velez Sarsfield, Buenos Aires, Sat 17 Jun, 3.40pm (local time)