An account is given of a viscometer which can be used for measuring the viscosity of synovial fluid when only small quantities are avaliable (0.15 ml) and taking into account the non-Newtonian character of the fluid. A technical description of the apparatus is given. A series of experiments are reported in which bovine synovial fluid was used to study whether and what changes took place in connection with storage, alterations in temperature, and post mortem. It was shown that storage at 4 degrees C for up to 2 weeks did not cause any particular changes, that alterations in temperature from room temperature to body temperature resulted in a moderate decrease in the relative viscosity, and that changes found 6 hours post mortem seemed to be insignificant. The conclusion drawn from the experiments is that the apparatus can be regarded as suitable for use in a more comprehensive study of the changes in viscosity which occur in various joint diseases.