Method for checking the supply of substitution fluid for an extracorporeal blood treatment apparatus, the extracorporeal blood treatment apparatus comprising: an extracorporeal blood circuit having an arterial blood line that leads from an arterial patient connection to a first chamber of a dialyser or filter that is divided into the first chamber and a second chamber by means of a membrane, and a venous blood line that leads from the first chamber of the dialyser or filter to a venous patient connection, a substitution fluid supply line for supplying substitution fluid at an arterial addition site upstream of the dialyser or filter, at which site substitution fluid is supplied to the extracorporeal blood circuit for predilution, and/or at a venous addition site downstream of the dialyser or filter, at which site substitution fluid is supplied to the extracorporeal blood circuit for post dilution, a blood pump for conveying blood in the extracorporeal blood circuit, and a bubble trap arranged in the venous blood line, characterised in that the part of the extracorporeal blood circuit located between the arterial patient connection and the arterial addition site of the extracorporeal blood circuit is filled with a fluid, in that a fluid is supplied to the extracorporeal blood circuit via the substitution fluid line connected either to the arterial or venous addition site until a preset fluid level has been reached in the bubble trap, in that the blood pump is actuated in order to convey a fluid in the extracorporeal blood circuit, and in that the fluid level in the bubble trap is monitored, a conclusion being drawn on a connection between the substitution fluid line and the arterial addition site if the fluid level in the bubble trap does not fall below a preset level or a conclusion being drawn on a connection between the substitution fluid line and the venous addition site if the fluid level in the bubble trap does fall below the preset level. an extracorporeal blood circuit having an arterial blood line that leads from an arterial patient connection to a first chamber of a dialyser or filter that is divided into the first chamber and a second chamber by means of a membrane, and a venous blood line that leads from the first chamber of the dialyser or filter to a venous patient connection, a substitution fluid supply line for supplying substitution fluid at an arterial addition site upstream of the dialyser or filter, at which site substitution fluid is supplied to the extracorporeal blood circuit for predilution, and/or at a venous addition site downstream of the dialyser or filter, at which site substitution fluid is supplied to the extracorporeal blood circuit for post dilution, a blood pump for conveying blood in the extracorporeal blood circuit, and a bubble trap arranged in the venous blood line, the part of the extracorporeal blood circuit located between the arterial patient connection and the arterial addition site of the extracorporeal blood circuit is filled with a fluid, in that a fluid is supplied to the extracorporeal blood circuit via the substitution fluid line connected either to the arterial or venous addition site until a preset fluid level has been reached in the bubble trap, in that the blood pump is actuated in order to convey a fluid in the extracorporeal blood circuit, and in that the fluid level in the bubble trap is monitored, a conclusion being drawn on a connection between the substitution fluid line and the arterial addition site if the fluid level in the bubble trap does not fall below a preset level or a conclusion being drawn on a connection between the substitution fluid line and the venous addition site if the fluid level in the bubble trap does fall below the preset level.