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Context: Anticoagulants have been used in New Zealand for decades, but few data are available on the sustainability of these toxins for rodent control. It is important to regularly monitor for resistance in long-term brodifacoum-use areas and establish a database for future references. Aims: This study aimed to estimate the effective dose (ED50) of brodifacoum for ship rats from an area of New Zealand with no history of brodifacoum use, in order to establish a blood-clotting response test for assessing resistance in rodent populations from other areas. Methods: A ranging study was conducted whereby successive groups of ship rats were administered brodifacoum doses that were increased or decreased progressively, until an International Normalised Ratio (INR) of 3.6 was reached. Linear regression was used to model the relationship between dose and INR, and ED50 dose was estimated using the resulting model. Results: None of the rats appeared susceptible to brodifacoum at previously reported LD50 exposures for this species. The ED50 of brodifacoum was estimated to be 2.88 mg/kg for males and 3.81 mg/kg for females. These values are 6–8 times greater than the previously published lethal dose values for ship rats in New Zealand. Conclusions: Blood-clotting inhibition was detected in the rats only following high doses of brodifacoum, which may indicate resistance within the sampled population. Implications: Relatively low susceptibility, or resistance, to brodifacoum in New Zealand ship rats may be mediated by spatial connections between areas with different histories and patterns of anticoagulant rodenticide use. In this study we determine the effective dose (ED50) of brodifacoum for ship rats from an area with no history of brodifacoum use to establish a blood-clotting response test for assessing resistance in areas where brodifacoum has been used extensively. The ED50 of brodifacoum was estimated to be 6–8 times the published lethal dose of brodifacoum in New Zealand, suggesting brodifacoum resistance may develop even in areas of negligible or low brodifacoum use that are spatially connected to other areas where these poisons are used. Photograph by Jaskaran Dhillon. [ABSTRACT FROM AUTHOR]