There is disclosed a method of distributing particles in a flow of gas and a needleless syringe for use in the needleless injection of particles into the skin or mucosa of a vertebrate subject. The syringe has the advantage that no bursting membranes are required, reducing noise, and that the particles are substantially uniformly distributed in the gas stream. This is achieved by providing a convergence which reduces pressure of the gas flowing in the gas flow path due to the Venturi effect such that particles initially located outside of the gas flow path are drawn into the gas flow path under the action of the reduced pressure and become entrained in the gas. An exit nozzle is provided to accelerate the particles so entrained. In another aspect of the invention, there is provided a method of creating a gas flow in a needleless syringe which comprises flowing gas through a first convergence into a chamber to form a transsonic gas jet in the chamber and passing the gas jet from the chamber into a second convergence and along the nozzle. This provides an advantageous flow regime suitable for the needleless injection of powders.