ABSTRACT The paper presents the main PETROBRAS observations obtained from its four year experience in performing more than 50 DSTs at over 500 m (1,640 ft) water depth, utilizing dynamic positioning units. Records and parameters obtained from DSTs carried out in Marlim and Albacora giant fields are also shown. The paper informs on a proven and available technology to test wells, in a safe way, in water depths up to 1,600 m (5, 250 ft) that can be extend this limit up to 2,500 m (8,200 ft). DRILL STEM TESTS EVOLUTION Formation tests evolution is associated to the development of the theory of fluid flow in porous media and the technological progress of oil industry. So, in a worldwide level, we passed from the tests performed onshore with packer and bottom hole retainer valve (1926) to the sophisticated tests in deep waters today. Puzzled with some unexpected results, the technicians of 1933 decided to run a mechanical pressure gauge with the test string, to verify if some' negative result observed on surface, for instance no flow, were caused either by formation or mechanical problems. During the fifties, in the, period when exploration and exploitation were concentrated onshore, mainly with vertical holes, the tester valves were operated by column rotation. In the early offshore exploration, in the mid fifties, the test strings did not undergo important alterations, because the wells kept on being verticals, being drilled by jack-ups. The only changes were produced on the surface well testing equipments, mainly burners and storage tanks. By economic reasons, the offshore exploitation philosophy changed in relation to that for onshore. Thus, fixed production platforms, with deviated wells, rendered the tester valves operated by string rotation into a problem. Then, tester valves operated by pipe reciprocation showed up in the early sixties. The use of floating rigs made those valves obsolete, forcing the outcoming of tester valves operated by annulus pressure in the early seventies, besides the mandatory use of slip-joints and subsea test trees. This phase in Brazil may be limited by the wells 1-RJS-9A (discovery of Campos Basin in december, 1974, at 124 m - 407 ft water depth) and 1-RJS-305 (discovery of Albacora main reservoir in November, 1984, at 437 m - 1,434 ft - water depth). The modern phase, characterized by the use of dynamic positioning rigs at water depths over 500 m (1,640 ft) and by new test procedures, has the well 1-RJS-219A as its initial mark in Brazil (Marlim discovery in February, 1985, at 853 m - 2,799 ft - water depth). During this phase, thirty three wells have been drilled making continuous domestic records on test results and on waterdepths.