A cuvette for taking up a fluid and mixing the fluid with a reagent for analysing the mixture consists of a body (10) of glass or polymeric material having a first cavity (12) in which the fluid can be taken up, preferably by capillary action, through an inlet (13), and at least one further cavity (21) exerting capillary force on fluid which is transported from the first cavity (12) into a reception cavity (17) by subjecting the cuvette to centrifugal force. The further cavity (21) preferably exerts capillary force through a wick (19) which does not extend as far as the bottom of the reception cavity (17), and a capillary channel (20). In those cases where more than one further cavity (21) is provided, each such cavity (21) communicates with a further reception cavity into which the fluid can be transported from the cavity (21) by the exertion of centrifugal force. The cuvette may also have cavities for receiving washing or diluting liquid, which are connected in series or in parallel with the cavity (12).