Анотация: The objective of the present study was to compare the efficacy of conventional and extended. - intramammary (IMM) therapy of persistent subclinical mastitis in lactating dairy cattle using nafcillinpenicillin-dihydrostreptomycin combination (NPD). Sixty-five dairy cows with 126 infected quarters were enrolled in the study. Infected cows were allocated randomly to 1 of 3 different treatment regimens: (1) conventional group: NPD administered IMM 3 times at 24-h intervals (20 infected cows, 43 intramammary infections [IMI]), (2) extended group: NPD administered IMM 6 times at 24-h intervals (23 cows, 43 IMI), and (3) untreated control group (22 cows, 40 IMI). The overall bacteriological cure (BC) rates for subclinical IMI were 86.04 percentage, 100 percentage, and 20 percentage for the conventional, extended and the control groups, respectively; indicating a higher BC rate (P<0.0001) for the treated groups than the control group. Significant difference (P=0.029) concerning the BC rate was also observed between the extended and the conventional groups. Significant difference (P=0.0021) in somatic cell count (SCC) was detected between the extended and the control group. Fat percentage increased in the conventional (P=0.029) and in the extended (P<0.0001) groups, and protein percentage increased only in the extended group (P=0.0016). There was no significant difference in posttreatment milk production between the groups (P>0.05). Results of this study indicate that NPD therapy was effective in eliminating subclinical IMI in lactating dairy cows, and that extended therapy enhanced BC rate and reduced SCC