For some two millennia Istanbul has been one of the world's greatest cities, and is today classified as an "Alpha-" world city in the Globalization and World Cities (GaWC) group's 2010 rankings. Istanbul is an emerging global city that is facilitating Turkey's transnational integration into the global economy, and as of June 2009 the city had the second largest office stock among all Southeast European countries. This paper investigates the determinants of office rent levels in the city's Central Business District (CBD) and the spatial variation of rents in a polycentric metropolis. The paper uses a stepwise regression and a Mamdani-type fuzzy rule-based model to estimate office space rents, and compares empirical results with those of a conventional OLS regression analysis. Rents are driven not only by physical characteristics and locational services, but also the terms of lease contracts. New CBD locations that command the highest rents on spacious, high-rise office spaces with comparatively few employees conform well with contemporary accounts of world-city financial districts, and confirm the significance of urban office infrastructures for globally oriented financial elites beyond the familiar roster of Global North world cities. The traditional center retains the core purpose identified nearly a century ago, in Marshall's analysis of centralization during the most vibrant period of the industrial age. Despite all the transformations of transnational urbanist polycentricity, the traditional CBD is still the place with the most diverse mix of activities, and the greatest variation in rents.