Venezuelan President Hugo Chavez has urged Iran to increase investment in his country's oil and gas assets.
Mr Ahmadinejad (right) described Mr Chavez as a "brother"
Visiting Tehran as part of a foreign tour, Mr Chavez also pledged Venezuela would "stand by Iran at any time and under any condition".
Iranian President Mahmoud Ahmadinejad described Mr Chavez as a "brother and trenchmate".
Venezuela and Iran are close allies, both members of the Opec oil-producing cartel and fierce critics of the US.
Iran faces growing pressure from the US over its nuclear programme and its support of the Hezbollah militia, currently battling Israeli forces in Lebanon.
Mr Chavez has been accused by the US of trying to destabilise South America, after he signed a deal to purchase arms from Russia earlier on his foreign tour.
Mr Chavez told reporters in Tehran that he hoped Iranian oil firm would invest in Venezuela's Orinoco region.
"We invite Iranian companies to the Gulf of Venezuela and the delta for gas production," he said.
He is expected to sign a number of commercial deals with Iranian firms during his visit.
Iranian investment in Venezuela last year was valued at $1bn, much of it directed towards energy, construction and engineering projects.
The delegation from Caracas includes Oil Minister Rafael Ramirez.
The Venezuelan leader said Iran and his country were firm partners in the fight against "imperialism".
Mr Ahmadinejad told state-run television Iran and Venezuela should "have to be supporters of each other".
"Chavez is a source of a progressive and revolutionary current in South America and his stance in restricting imperialism is tangible," he was quoted as saying.
Mr Chavez also held talks with Iran's Supreme Leader, Ayatollah Ali Khamenei.
Mr Chavez had been expected to arrive in Iran on Friday, his birthday, but the visit was postponed by a day. No reason was given for the delay.
Mr Chavez is on a foreign tour that has until now taken him to Russia and Qatar.
After Iran, he is expected to visit Vietnam and Mali.