The bearing of Mesozoic geology on the time and character of the opening of the North Atlantic is investigated. Of the alternative initial fits of the continents, that along the boundaries of the Quiet Magnetic Zones is preferred. Igneous and tectonic activity along the continental margins, the nature and distribution of epicontinental sediments, and several features of fossil distribution are briefly reviewed and the following interpretation tentatively proposed. Although some tension and subsidence occurred in the late Triassic, it is believed that the continental fragments did not begin to move apart until the Jurassic. Based mainly on the collapse of the Mediterranean carbonate platform, faunal divergence between Europe and Africa, and igneous episodes in the Atlantic region-the earliest likely date of opening of the southern North Atlantic is late Lower Jurassic, although it could possibly have been late Upper Jurassic, a time of tectonic disturbance on the eastern margins. Rifting extended northward in the Cretaceous to separate the Rockall Bank from the British Isles, and a probable accelerated rate of sea-floor spreading in the Upper Cretaceous is associated with increased tectonic and igneous activity, collapse and detachment of parts of the continental margins, and opening of the Bay of Biscay. Cenozoic spreading in the northern North Atlantic was preceded by a long period of subsidence and episodic tensional faulting. Eustatic sea level rises, notably in the late Cretaceous, may relate to oceanic ridge uplift consequential upon accelerated sea-floor spreading rates. Possible causes of crustal thinning in regions of continent margin subsidence are discussed. Subsidence prior to separation may result from crustal thinning under tension. Limited evidence for uplift close to the rifting axis, and stronger evidence for marginal subsidence after rifting, is held to support the Vogt-Ostenso model based on analogy with the East African rift system. Tensional effects associated with continental drift are, to variable extents, dispersed in both space and time.