Venezuelan President Hugo Chavez has held talks in Tehran with Iranian counterpart Mahmoud Ahmadinejad.
The two leaders are vocal critics of the US
The two leaders were expected to sign a series of co-operation agreements in the areas of energy and industry.
It is the Venezuelan leader's third visit to Iran in two years, and follows trips to Russia and Belarus.
Venezuela has been a strong supporter of Iran's nuclear programme, in defiance of Western powers, and the two nations are both major oil exporters.
Mr Chavez arrived in Tehran on Saturday, calling the Iranian leader his "ideological brother", AFP news agency reports.
On Monday, the two leaders were due to oversee the beginning of construction of a joint petrochemical plant in the southern city of Asaluyeh, Iran's Irna news agency reported.
A similar plant is to be built in Venezuela
Mr Chavez's visit comes as Iran is facing the threat of toughened sanctions from the UN over its refusal to freeze uranium enrichment.
The US and EU accuse Iran of enriching uranium as part of a nuclear weapons programme, but the Iranians say its activities are for peaceful energy production.