A method is proposed for calculating the pressure and flow of air in a single perforated crop ventilation duct of constant cross-section and open area and embedded in a crop of variable depth. The concept of an effective depth is introduced to give economy in the calculation of crop resistance at successive points along the duct. The final solution is the balance of crop resistance against duct static pressure resulting from changes in frictional and velocity heads. Experimental work is required to measure regain coefficients and friction factors in perforated ducts under blowing and sucking conditions.