A morphogenetic study, from the first-leaf stage onwards, was made of barley plants grown at a range of plant populations from 50 to 1600 plants m-2. Increasing density reduced leaf number from 10.2 to 8 and caused stem internode elongation to start earlier at a lower node. Final stem length was reduced due to earlier cessation of growth. The lamina and sheath length of the lower leaves was increased with increasing density, and lamina width was decreased. At the highest density the apex reached the double ridge stage 6 days earlier than at the lowest density, and this difference persisted throughout the development of the apex. The rate of primordium production was little affected by density, but the duration was markedly affected. Primordium production stopped abruptly, first at the highest density, and then successively at the lower densities. At this stage changes were seen at the tip of the apex, and eventually the 10 distal primordia died. There were small differences in the length of shoot apex or ear during growth, but earlier cessation of growth in the higher densities led to a shorter ear. Growth of the internodes of the floral apex started first at the high density. These observations suggest that plants grown at high densities had a high concentration of gibberellins (GA) in their tissues. The promotion of apex development by the higher concentration of GA gave rise to earlier competition for nutrients diffusing through the apex. This lead to starvation of the tip of the apex and earlier death of this region and, consequently, fewer spikelets. Differences in the light environment of the plants, either intensity or red/far red ratio, most probably brought about the differences in GA.