A sterilized polyethylene intramammary device (IMD) was inserted through the papillary duct into a front and a rear mammary quarter of 12 primiparous dairy cows, which were mastitis-free. This study was conducted during lactation from month 1 postpartum to completion (generally month 9). After the device was inserted, milk somatic cell counts (SCC) increased. Milk scc in the quarters fitted with IMD from foremilk, primary, and stripping samples were 5.2, 2.6, and 5.9 X 10(5) cells/ml, respectively. Comparable counts for control quarters were 2.1, 1.4, and 3.6 X 10(5) cells/ml. The SCC in foremilk from the quarters fitted with the device were 2 to 3 times greater than the counts in foremilk from control quarters during month 2 and subsequent months of lactation. The SCC were significantly increased in primary and stripping samples from quarters fitted with IMD; however, the SCC of 2.6 X 10(5) cells/ml in primary milk would not affect market milk quality. Milk production for the lactation was decreased (0.6 kg/quarter/day) in the quarters fitted with IMD in comparison with control quarters throughout the lactation. There were no differences in percentages of milk fat or protein between deviced and control quarters. Conductivity values were increased in foremilk and stripping samples, but not in primary samples, for the quarters with IMD. Bovine serum albumin concentrations were also increased.