The gaseous messenger nitric oxide (NO) contributes to biological effects of oestrogen in target tissues, including reproductive organs, bone, cardiovascular and central nervous systems. Vasodilation and anti-atherosclerotic properties of NO have been shown to play a role in these effects. The possibility that NO acts also through regulation of the signal transduction cascade triggered by oestrogen, instead, has never been investigated. To study this we have used the MCF-7 human breast cancer cell line, an established model for oestrogen signalling. Exposure of these cells to 17-beta-oestradiol (E(2)) in the presence of NO gave rise to activation of signalling events additional to those triggered by E(2) alone, namely tyrosine phosphorylation of specific proteins, including the insulin receptor substrate-1, with recruitment to this adapter of the phosphatidylinositol 3'-kinase and persistent activation of Akt (protein kinase B). Active Akt, in turn, prevented E(2) from activating p42/44 extracellular signal-regulated kinases (ERK 1/2). These effects of NO, which were mediated through generation of cyclic GMP and activation of the cGMP-dependent protein kinase I, initiated in the first minutes after administration of oestrogen. The consequences, however, were long lasting, as modulation of Akt and ERK 1/2 activities by NO was responsible for inhibition of E(2)-triggered cell growth and regulation of oestrogen responsive-element dependent gene transcription. Generation of NO is stimulated by both E(2) and growth factors known to contribute to the complex network of intracellular events regulating the biological actions of oestrogen. It is conceivable, therefore, that modulation by NO of E(2) early signalling, here described for the first time, has broad significance in regulating cellular responses to the hormone.