The control of haematopoietic colony‐stimulating factors (CSF) gene expression by interleukin 1 (IL‐1) and tumour necrosis factor alpha (TNF‐alpha) in cultured endothelial cells was studied by RNA hybridization and nuclear gene transcription. Both IL‐1 and TNF‐alpha induced, with somewhat different kinetics, a slow but marked accumulation of granulocyte‐macrophage (GM)‐ and granulocyte (G)‐CSF mRNAs in endothelial cells; macrophage (M)‐CSF mRNA increased more rapidly but more moderately. Simultaneous treatment with maximally stimulating concentrations of both IL‐1 and TNF‐alpha had an additive effect on the accumulation of the three mRNAs, suggesting that both mediators act via independent pathways. The mechanism of CSF mRNA accumulation in endothelial cells was explored by nuclear run‐on experiments, which showed that both IL‐1 and TNF‐alpha increase GM‐CSF, G‐CSF and M‐CSF gene transcription to varying degrees.