A method for controlling transmission of messages in an interactive television system comprising a network and a plurality of receiver units connected to the network; the method comprising: transmitting to the receiver units an trigger inviting a response from each receiver unit and indicating that a delay procedure is to be applied to the responses; and at each receiver unit: accepting from a user of the receiver unit data in response to the trigger; forming a response message for transmission to the network in response to the trigger, the response message including the data received from the user; determining the length of a delay interval to be applied to the response message; and transmitting the response message to the network after the delay.