Influence of a treatment of subclinical mastitis on electrical conductivity of milk before let-down

10 cows (German Brown x Brown Swiss) with 29 infected quarters (14 Staphylo coccus aureus, 10 esculin positive streptococci, 5 Staphylococcus spec.) we re treated daily after morning milking over a period of 4 days. Ampicillin and cloxacillin (500 mg each per dose) were administered to all 40 quarters intracisternally. During the evening milkings on 4 test days (day 3 before treatment, day 1 of treatment, days 5 and 9 after the last treatment), ele ctrical conductivity based on 25 degreesC (EC) was measured in fore milk. T o avoid ejection of alveolar milk, EC measurement was the first contact to the udder. Independent of the initial level, EC of all quarters was elevate d by 2.1 mS/cm on average at the first milking following the first treatmen t. 9 days after the end of treatment, EC had declined to the initial level in all quarters classified as mastitic, non specific and latent infected be fore treatment. By contrast, EC values of healthy quarters remained elevate d compared to their initial level (p < 0.01). This might be due to increase d cistern tissue permeability caused by mechanical and pharmacological effe cts of antibiotic treatment. Despite the fact that somatic cell count and t otal bacteria count were reduced by treatment (p < 0.01), results showed th at EC measurement on the first days following treatment was unsuitable for checking the success of treatment.