A method of and apparatus for controlling the velocity of a stream of droplets in a continuous ink jet printing system comprises controlling, from a system cabinet, the velocity of the stream expelled from the print head through a nozzle under pressure from a pressure source, in dependence upon a measured pressure of the ink in accordance with a predetermined relationship between the velocity and the pressure. On start up the system calibrates for the pressure differential P.sub.c due to the conduit length and the relative elevation of the print nozzle; and a determination of the ink viscosity in made at predetermined times. Thereafter the velocity is controlled in dependence upon a required pressure value P.sub.r in accordance with a stored look-up table, the required pressure value at any time being determined substantially by a given relationship.