Monitoring of ruminating activity in cows can help detecting deficiencies in structural fibre in the diet and also identifying related rumen disorders at an early stage. The aim of this study was to evaluate a noseband sensor system (NSS; "RumiWatch", Itin+Hoch GmbH, Liestal, Switzerland) under variable feeding regimens. Visual and electronic measurements of ruminating activity were compared in eight Holstein cows fed either roughage only diet, transition diet from roughage to concentrate, or constant feeding of 65% concentrate (dry matter basis). Increasing the concentrate amount in the diet lowered (P < 0.01) all ruminating parameters, regardless of the measurement method. The analysis showed that NSS overestimated (P <= 0.01) several chewing variables such as ruminating chews per minute, chews per bolus and ruminating chews per 10 min; however, total duration of rumination did not differ (P=0.49) between visual and NSS method. Also, the concordance correlation coefficient (CCC) analysis, representing the reproducibility of the NSS measurements, revealed a substantial agreement between visually observed and with NSS recorded values for ruminating chews per 10 min (CCC=0.92) and a high agreement for the number of boli per 10 min (CCC=0.82). In contrast, the reproducibility of NSS was moderate (CCC=0.67) for chews per bolus and low (CCC=0.36) for chews per min. In conclusion, the NSS is a suitable device to replace time-consuming visual observations for measuring total rumination time in cows under variable feeding conditions. However, specific NSS measurements such as chews per minute, chews per bolus, and the number of boli need further adjustements.