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A three-judge panel of the Court of Appeal said "there is no dispute that methylmercury is a reproductive toxin that can harm a developing fetus" and cause birth defects.
But the court said there was substantial evidence that at least some of the mercury found in tuna comes from natural sources, such as from rainfall depositing atmospheric mercury in the oceans and possibly from vents on the ocean floor.
The panel said that mercury in tuna therefore did not fall within the jurisdiction of Proposition 65, a consumer law passed by state voters in 1986. The measure mandates warnings on products that cause birth defects or cancer.
The panel ruled in a lawsuit filed by the state against the nation's three largest tuna companies in 2004.
The three companies are Tri-Union Seafoods LLC, maker of Chicken of the Sea; Del Monte Corp., maker of Starkist; and Bumble Bee Seafoods LLC, maker of Bumble Bee tuna.
The state had sought warnings to pregnant women and women of childbearing age. It said the messages could be provided through tuna can labels, grocery store displays or a public education campaign.
The appeals court upheld a finding by San Francisco Superior Court Judge Robert Dondero in 2006 that the mercury in tuna is naturally occurring and therefore not subject to Proposition 65 warnings.
The ruling could be appealed further to the California Supreme Court. A spokesman for state Attorney General Jerry Brown was not immediately available for comment on a possible appeal.
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