The physical parameters of oats of prime importance in determining milling quality are kernel content, the content of free kernels, hullability and screenings. In addition to screenings, hectolitre weight is used in the commercial trading of grain as an indicator of quality due to the absence of suitable methods, for commercial use, for determining the parameters that are closely related to milling quality. The effects of nitrogen (0–200 kg/ha), seed rate (200 and 300 seeds/m2) and plant growth regulator, chlormequat chloride, on grain quality were investigated at two locations, in one spring and three autumn sown experiments, in 1997/98 and 1998/99. Yields increased at higher rates of nitrogen and at the higher seed rate but were not significantly affected by chlormequat chloride. The effects of nitrogen rate and seed rate on kernel content were inconsistent but overall there was a small reduction at the higher nitrogen and higher seed rates and where chlormequat chloride was applied. In general, free kernels were reduced at the higher rates of nitrogen, at the higher seed rate and where chlormequat chloride was applied. Hullability improved at higher rates of nitrogen but was poorer at the higher seed rate and with chlormequat chloride. Screenings increased and hectolitre weight declined at the higher rates of nitrogen, the higher seed rate and with chlormequat chloride. The decline in hectolitre weight due to treatments within each experiment in general was associated with reductions in kernel content, content of free kernels and an increase in screenings. However, variation in hectolitre weight did not reflect variation in kernel content, free kernels and screenings between experiments. Overall, the changes in quality due to nitrogen rate, seed rate and chlormequat chloride were small compared with differences between varieties.