Perennial ryegrass (Lolium perenne L.) cv. S23 was exposed to 0, 50, and400 μg m− 3 SO2 for a 29 d period, harvested, and then exposed under the same regime for a further 22 d period of regrowth. Leaves from plants representing each exposure concentration were photosynthetically fed 14CO2 for 5 min at the end of each period. A significant increase in photoassimilation of 14CO2 and retention of I4C, concomitant with significant decreases in [14C]glycine and [14C]serine with increasing SO2 concentration, implied that there was an inhibition of the photorespiratory pathway. At the second harvest, leaves from plants exposed to 400 μg m− 3 SO2 also exhibited significant increases in [14C]sucrose and [14C]fructose.