Trends towards power saving and environmental friendliness essentially require light parts. An ideal method to reduce weight is foaming. Polybutylene terephthalate (PBT) has excellent thermo-mechanical and electrical properties, as well as outstanding dimensional stability at elevated temperatures. Therefore PBT is used as compact material in automotive and electrical applications. However, it has a major drawback: high flammability. This issue is even more important for PBT foams. To counter that, flame retardants are added. To maintain good structural properties of PBT foams, fine foam morphologies are required. However, foaming of semi-crystalline polymers is challenging due to a very narrow processing window. To assess the processing window, extensive knowledge of rheological properties is of utmost importance, since elasticity and viscous properties at processing conditions, namely high pressures and CO2 as blowing agent, determine the foaming behaviour. To quantify those influences at shear-rates relevant for processing conditions during foaming, a special in-line rheometer was developed. A die dedicated for rheological measurements was attached to a tandem extruder. The rheological properties of neat and flame-retarded PBT were investigated. It was found that introduction of halogen-free flame-retardant increases thermal, pressure and blowing agent effects on the melt. However, pressure effect was decreased by addition of blowing agent. An increase of CO2 was also observed from pressure dependent measurements under gas-loading. A comparison to long chain branched Polypropylene was also done: at high shear-rates PBT exhibited higher viscosity than LCB-PP, at low rates the reverse effect was found. This highlights the difficulties encountered during foaming: low viscosity at deformation rates relevant for bubble growth and thereby too fast expansion leading to not favoured foam morphology.