There is growing evidence that the South American Low-Level Jet is an important component of the water budget of the Amazon and La Plata Basins. Recently the large mesoscale convective systems (MCSs) in subtropical South America have been linked to strong episodes of the SALLJ. The TRMM satellite data have recently demonstrated that northern Argentina, Paraguay, Uruguay, and southeastern Brazil get a higher proportion (> 80 percent) of their annual rainfall from MCSs than any other place between 36 N and 36 S. The jet provides very moist air from the Amazon Basin and contributes to the strong low-level wind shear that is a necessary condition for long-lived MCSs. During January and February 2003, an international field program will investigate the structure of the SALLJ with widespread soundings, pibals, and a NOAA WP-3D research aircraft. One of the objectives of the research missions is to measure the properties of the SALLJ on occasions when it is apparently feeding the northern flanks of a large MCS. This would be the first data set with in situ and airborne radar data in this region, and this presentation will report on the degree of success in meeting these objectives, and hopefully present some quick-look data.