Method for injecting a liquid sample in an HPLC analyzing device, using an apparatus which comprises a first liquid circuit, a second liquid circuit, a valve assembly of the stator/rotor type, and a sample loop, wherein the first circuit and the second circuit are connected with one another via the valve assembly which comprises a first valve element and a second valve element co-operating with one another and rotatable relative to one another about a rotation axis for switching between different valve positions of the valve assembly, and wherein both ends of the sample loop are connected to the valve assembly such that in a first valve position and in a second valve position of the valve assembly, respectively, the sample loop is either part of the first circuit or part of the second circuit, respectively, said first valve element comprising a first valve inlet connected in the first circuit to sample supply means, a first valve outlet connected in the first circuit to waste discharge means, a second valve inlet connected in the second circuit to mobile phase supply means, a second valve outlet connected in the second circuit to the analyzing device, a first valve terminal connected to one end of the sample loop, and a second valve terminal connected to the other end of the sample loop, said second valve element comprising a first connection chamber, a second connection chamber and a third connection chamber, wherein in the first valve position the first chamber connects the second valve inlet with the second valve outlet, the second chamber connects the second terminal with the first valve inlet, and the third chamber connects the first valve outlet with the first valve terminal, while in the second valve position the first chamber connects the first valve terminal with the second valve inlet, the second chamber connects the second valve outlet with the second valve terminal, and the third chamber connects the first valve inlet with the first valve outlet; the method comprising the consecutive steps of: - initializing the apparatus such that the valve assembly is in the second valve position, and that in this position an initial quantity of the sample is present at least in a section of the first circuit between the sample supply means and the first valve inlet, and that a mobile phase is present in the second circuit, while the liquid pressure in the second circuit is higher than the liquid pressure in the first circuit; - switching the valve assembly from the second valve position into the first valve position; - supplying an additional quantity of the sample into the first circuit to such an extent that the sample loop is at least partly filled with the sample; - switching the valve assembly from the first valve position into the second valve position, as a result of which the sample in the sample loop is contained in the second circuit; and - injecting the sample together with the mobile phase from the second circuit into the analyzing device; characterized in that said first valve element comprises a third valve outlet and said second valve element comprises a fourth connection chamber such that during switching of the valve assembly from the second valve position into the first valve position the valve assembly temporarily assumes an intermediate valve position inbetween the second and the first valve positions, in which intermediate valve position the fourth chamber connects one of the first and second valve terminals (3, 6) with the third valve outlet, while in that intermediate valve position the sample loop is neither part of the first circuit nor part of the second circuit for allowing decompression of the sample loop via the fourth connection chamber and the third valve outlet. - initializing the apparatus such that the valve assembly is in the second valve position, and that in this position an initial quantity of the sample is present at least in a section of the first circuit between the sample supply means and the first valve inlet, and that a mobile phase is present in the second circuit, while the liquid pressure in the second circuit is higher than the liquid pressure in the first circuit; - switching the valve assembly from the second valve position into the first valve position; - supplying an additional quantity of the sample into the first circuit to such an extent that the sample loop is at least partly filled with the sample; - switching the valve assembly from the first valve position into the second valve position, as a result of which the sample in the sample loop is contained in the second circuit; and - injecting the sample together with the mobile phase from the second circuit into the analyzing device;