Methanolic extracts of 18 Thai plants from Doi Tung, the north of Thailand, were examined for brine shrimp lethality and antioxidative activity using 1,1-diphenyl-2-picrylhydrazyl (DPPH) assay. In brine shrimp lethality, the highly toxic plant extracts were Mitrephora wangii (LC(50) 14.8 µg mL(-1)) and Hydrocotyle javanica (LC(50) 13.3 µg mL(-1)). The extracts of Sageretia cordifolia, Ulmus lancaefolia and Acer chiangdaoense exhibited highly antioxidative activities with IC(50) values of 6.7, 8.1 and 9.8 µg mL(-1), respectively. Moreover, the extracts of S. cordifolia, U. lancaefolia and A. chiangdaoense have shown slightly synergistic effects (combination index; CI < 1) with L-ascorbic acid on DPPH radicals. A number of plants demonstrated either brine shrimp lethality or free radical scavenging activity.