The Cumbernauld-based maker of Irn Bru, Tizer and Rubicon posted a 1.1% fall in sales for the 15 weeks to 9 May.
It also came up against tough comparisons from a year earlier, when it sponsored the Glasgow Commonwealth Games.
But the business said sales in the wider soft drinks market lifted by 0.7% in the same period.
Barr's sales figures excluded its offloaded Orangina and Findlays water coolers drinks brands, but did include a Â£3m revenue contribution from its recently acquired Funkin cocktail drinks business.
The firm said it would continue to extend its reach into the south of England by developing its factory in Milton Keynes.
It plans to extend the site's warehousing capacity, and will buy more land next to the plant at a cost of Â£11m.
In a statement ahead of its annual general meeting, AG Barr said: "Margins continue to hold up well as we drive efficiency improvement across the business.
"We have a strong summer brand programme planned across all of our core brands and expect to see a return to sales growth in the second half of this financial year."
Soft drinks firm AG Barr has reported flat sales amid a "competitive and volatile" market.