Changes in the gamma-butyrolactone and pantolactone contents in yeast cells and musts during fermentation and subsequent "flor" veil formation of Sherry wines were studied. Saccharomyces cerevisiae race cerevisiae, S. cerevisiae race bayanus and S. cerevisiae race capensis were used. During the alcoholic fermentation (first 31 days), gamma-butyrolactone contents in musts and yeast cells were similar for the three yeast races tested. In this period, pantolactone was excreted to the must by Bayanus and capensis races, and it was not detected in cerevisiae race cells. During "flor" veil formation (31 to 134 days), bayanus and capensis races yield higher gamma-butyrolactone and pantolactone contents than cerevisiae race in the wines. In the final wines, pantolactone contents were always lower than those of gamma-butyrolactone.