Much research has been done on maximising boll set to produce maximum lint yields, but there is need to develop sufficient plant structure to support the extra boll load. There is also the necessity to achieve better nutrient balance and high fertiliser-use efficiency, and overcome yield barriers. Plant growth regulators give producers the flexibility to modify plant growth to suit current growing conditions to maximise benefits. A field experiment was conducted at the Agricultural Research Center, Giza, Egypt, during the 1999 and 2000 seasons The aim was to study the effects of nitrogen (N) fertilisation (at 95 or 143 kg N ha-1), foliar application of potassium (at 0.0, 319, 638 or 957 g K ha-1, applied twice: 70 and 95 days after planting) and the plant growth regulator (PGR) mepiquat chloride (MC) (applied twice; 75 days after planting at 0.0 (control) and 48 g active ingredient (a.i.) per ha, and 90 days after planting at 0.0 (control) and 24 g a.i. per ha) on yield of Egyptian cotton cultivar Giza 86. Seed cotton yield per plant and seed cotton and lint yield per ha were increased at the 143 kg N ha-1 rate and with foliar application of 319 g ha-1 K and 48 + 24 g a.i. ha-1 MC. In the absence of N × K, N × MC, K × MC, and N × K × MC interactions, N applied at 143 kg ha-1, K at 319 g ha-1 and MC at 48 + 24 g a.i. ha-1 should be used to improve cotton growth and yield of Giza 86.