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Heart Gold and Soul Silver: In-Game Trades
There are some in-game trades that allow you to get rare held items and Pokemon with special moves. As a bonus, traded Pokemon get an experience bonus so they gain level faster than normal. The Pokemon you receive from a trade has the same level of the Pokemon you traded for it.

Trade:
For:
Bellsprout
Bellsprout
Onix
Onix
This trade is available inside the southern-most house in Violet City, near the Pokemon Center.
Holding:
Persim Berry

Trade:
For:
Drowzee
Drowzee
Machop
Machop
This trade is on the 5th floor of the Goldenrod Department Store.
Holding:
Macho Brace

Trade:
For:
Krabby
Krabby
Voltorb
Voltorb
This trade is in a house north of the Poke Mart in Olivine City.
Holding:
Cheri Berry

Trade:
For:
Dragonair
Dragonair
(Female)
Dodrio
Dodrio
This trade is in a house northeast of the Pokemon Center in Blackthorn City.
Holding:
Smoke Ball

Trade:
For:
Dugtrio
Dugtrio
Magneton
Magneton
A scientist in the Power Plant.
Holding:
Metal Coat

Trade:
For:
Haunter
Haunter
Xatu
Xatu
This trade is in the Pewter City Pokemon Center.
Holding:
Wacan Berry

Trade:
For:
Any Pokemon
Steelix
Steelix
After you defeat Jasmine in a rematch you can find her in her Gym in Olivine to trade any Pokemon for her Steelix.
Holding:
Soothe Bell

Trade:
For:
Pikachu
Pikachu
Pikachu
Pikachu
After you defeat Lt. Surge in a rematch he will be in the Magnet Train Station in Saffron City ready to trade his French Pikachu.
Holding:
Yellow Shard

Trade:
For:
Bonsly
Bonsly
Rhyhorn
Rhyhorn
After you defeat Brock in a rematch you can find him in Diglett's Cave to trade with him.
Holding:
Passho Berry

Trade:
For:
Forretress
Forretress
Beldum
Beldum
This trade is available after you beat Red, get a Hoenn starter (Treecko, Torchic, Mudkip) from Steven at Silph Co, and talk to Steven at the Pewter City Museum. You will need to go to Silph Co. and find Steven in the lobby to trade with him.
Holding:
Dawn Stone