A method of detecting a fluid influx in a well being drilled with a drill string having a drilling fluid circulated therethrough from a tank via a supply line system connected to the drill string, the method being used while connecting or disconnecting a pipe to or from the drill string and comprising the steps of:a) ceasing circulation of the drilling fluid;b) disconnecting the supply line system from the drill string so as to allow any drilling fluid in the system to flow back into the tank while connecting or disconnecting a pipe;c) reconnecting the supply line to the drill string and recommencing circulation of the fluid;d) while repeating a)-c) during the normal course of drilling the well, monitoring the change in level of fluid in the tank so as to determine a model of the change in level in the tank with respect to time during steps a)-c); ande) comparing the change in tank level observed when subsequently performing steps a)-c) with the change predicted by the model so as to detect a difference therebetween indicative of an influx.