Experimental investigations of the pattern of Middle Cretaceous oceanic surface currents in the Northern Hemisphere have been made, using planetary vorticity models developed by Von Arx. The main features of the ocean configuration are an open Tethys seaway and Central American region, an Atlantic Ocean approximately 50 percent of its present size, a Pacific Ocean about 25 percent larger than it is at present, and the ancient Tethys Ocean. Two zonal wind profiles have been considered in the experiments, a “glacial” profile simulating the zonal profile in existence today and a “nonglacial” profile with the westerlies belt about 10° farther north than the glacial profile. Paleoclimatic data support the existence of the non-glacial situation in the Middle Cretaceous, and these results are considered most relevant. The features of this experiment which differ significantly from the present-day oceanic surface circulation are the circum-global flow of the Tethys current, the flow of the Gulf Stream into the Labrador Sea, and cross-polar flow of water from the Atlantic into the North Pacific. In another experiment, the Tethys region was closed off near Malaysia in order to simulate the obstruction of the Tethys current. The Central American region was left open. Weak and variable currents are seen in the Tethys (or ancient Mediterranean), and the circum-global current is absent. North Atlantic circulation occurs from Greenland down to the equator. Atlantic and Pacific waters exchange through the Isthmus of Panama. A net inflow of Pacific water through this gap into the Atlantic maintains the cross-polar flow of “warmer” waters from the Atlantic to the Pacific. Geologic evidence of the first land connections in the Isthmus region can be correlated with the formation of the Labrador Current (3.4 m.y. ago and 3 m.y. ago, respectively). It is proposed that cessation of net inflow of Pacific water into the Atlantic caused cross-polar flow to stop. This, coupled with a deteriorating mean global temperature, was sufficient to trigger the formation of ice in the Arctic region.