We investigate the separate effects on metropolitan patenting of inventor agglomeration and the structure of social networks linking inventors within and across metropolitan areas in the United States between 1977 and 2002. Using patent data we have been able to assign a metropolitan location to individual inventors, link inventors who have co-authored patents, and characterize the structural features of the networks linking inventors. Our main question concerns the relative importance of salient features of the social networks linking inventors on metropolitan patenting productivity. We find that agglomerative features of metropolitan areas are more important determinants of metropolitan patenting productivity than structural feature of the inventive networks. While the aggregation of isolated inventors correlates positively with patenting productivity, density of connections correlates negatively.