Wind activated rooftop ventilators have recently been rediscovered as appropriate energy conserving alternatives and/or supplements to power ventilation. Designers of passive solar heating/ cooling systems, for instance, often use ventilators in various ways as important components of their systems. Unfortunately, results with present types of ventilators are often less positive than expected. In recent wind tunnel tests the commonly available turbine type ventilator, in a 10 mph wind, moved only 20% more air than an open stack with no ventilator at all. Given the turbine's poor showing, the development of a ventilator with significantly improved performance was seen as feasible and was adopted as the focus of this project. An extensive literature search revealed several design principles established by others in comparative tests dating as far back as 1842, and the existence of over 300 patented ventilators designs. For this project a speculative ventilator design was evolved incorporating (1) criteria established after preliminary tests, (2) the above mentioned design principles and (3) three new concepts. Eight 1/3 scale ventilator cowls were produced in molded fiber reinforced plastic and altered in various ways in an attempt to optimize performance. Also tested were a variety of simple forms and three experimental types that had previously been tested at a larger scale. The models were tested in a 3-foot diameter open-throat wind tunnel on a 4-inch diameter air shaft or "stack." Results are given in stack velocity, volumetric flow, and percentages of enhancements over an open stack. The percentage enhancement values allow ventilator tests done at different scales to be compared. / M.Arch.