But the company said reports that it may shut the brewery are "pure speculation". David Gosnell, managing director of Diageo Global Supply, said: "This assessment is at a very early stage and will continue into 2008 before reaching a conclusion.
"Diageo fully recognises the huge importance of St James's Gate Brewery in the history of Guinness and Dublin, and this important aspect of our brand and heritage will be fully embraced in the assessment."
St James's Gate has been brewing since before 1979, when Arthur Guinness took over a small, disused and ill-equipped brewery on the site and started developing what would become the world's best-known stout. The 56-acre site is the largest stout-producing brewery in the world, and has been revamped and modernised with new technology and capital investments worth some £240 million.
Diageo still has no plans to launch Guinness Red in the off-trade - but may make the decision in September. "It is still being tested in the on-trade and Diageo is pleased with how the trial is going,"
a spokeswoman told OLN.