This project aimed to examine the relationship between spikelet primordium size at the double ridge stage and the final grain weight in two row barley. Earlier work in this laboratory had suggested that the size of primordia at the double ridge stage influenced final grain size. This work was repeated to confirm the finding and, whilst the relationship held, the results were quantitatively different due to differing environmental conditions during the later stages of grain development. Subsequent work sought to manipulate primordial width at double ridge stage by altering both temperature and nitrogen supply in order to determine the generality of the apparent primordium width/grain weight relationship. Results from these experiments led to investigation of the effect of tillers and the application of a growth retardent on the size of spikelets and grain. Lowered nitrogen supply for the first 40 days after sowing reduced the size of spikelet primordia at the double ridge stage and this resulted in the production of smaller grain. Reduced nitrogen supply for only 20 days after sowing led to reduced grain weight in cv. Maris Mink but not in cv Proctor. Proctor had larger central florets and set larger grain than Maris Mink. Reduced nitrogen supply during the vegetative stage of mainstem apex development had no effect on spikelet size in Proctor but marginally increased spikelet size in Maris Mink. Final grain size was reduced in Proctor but increased in Maris Mink. Lowering the temperature in which plants were grown increased grain size and number but had no effect on spikelet size.