In this study the rheological properties of two electrorheological (ER) fluids (experimental samples) under the influence of an AC as well as under DC conditions are investigated using a rotational viscometer. Plastic-fluid behavior quite well describes the flow curves with a yield stress which depends upon the applied electric field. The results show that the behavior of an ER-fluid depends on its composition. Comparison between the shear-type (rotational viscometer) and flowtype (slit flow) measurements leads to the hypothesis that the inhomogenous electric field (in viscometer) may be the cause for an increase of the measured ER-effect relative to that measured in a homogeneous field (channel with smooth electrodes). Based on this hypothesis, the ER-effect in slit flow should increase by altering the electrode morphology. For that purpose, the effects of the electrodes morphology on the slit flow of an ER-fluid are studied via laser-Doppler-anemometry (LDA). Two types of grooved electrodes were used. In the presence of an AC-field, the ER-effect with grooved electrodes is greater in comparison to the case of smooth electrodes at the same field strength and pressure drop. An average factor of 2.5 describes this increase. This increase in the ER-effect is accompanied by a decrease in electric current. Under a DC-field the ER-effect with grooved electrodes is somewhat better than the smooth electrodes but only at very low field strength. Increasing the field strength leads to a decreased ER-effect in comparison to the smooth electrodes.