A pressure-actuated on-chip injection system has been developed that is compatible with shallow microchannels with a very large aspect ratio, i.e. 1 mm deep and up to 1000 mm wide. Such channels offer potential advantages in the miniaturisation of liquid chromatography and other separation methods as they allow high loadability and low sample dispersion at the same time. Computational fluid dynamics simulations were performed to predict the flow profiles and the transport of a sample in the system and to justify the injection principle. Based on these simulations, a prototype integrated into a chip for hydrodynamic chromatography has been realised and tested experimentally. The performance of the device is satisfactory and the results are in qualitative agreement with the numerical models.