NDMA is formed in malt because NOx reacts with certain amines in germinated barley when it is kilned. Hordenine is the major precursor of NDMA, although gramine and sarcosine can possibly contribute minor amounts. The hordenine is formed in the developing seedling. The amount of hordenine present in unkilned malts, made using a variety of malting techniques, is always much more than sufficient to account for NDMA formation. Malting techniques which inhibit hordenine breakdown during kilning decrease NDMA formation. Nitrosated intermediates formed from hordenine on treating with NOx at relatively low temperatures (ca 40 degrees C), can be separated by HPLC. They give NDMA on heating with additional NOx. The conditions under which the intermediates are formed and under which they are converted to NDMA correspond to the observed pattern of NDMA formation during kilning. The effects of sulfuring at different times can be explained by the effects on formation and breakdown of the intermediates.