The sensitivity of the early life stage (ELS) toxicity test for two compounds with different modes of action was determined, and related to other toxicity tests with the same compounds. The zebrafish, Danio rerio, was used as a test organism, and the two model compounds were 1,2,3-trichlorobenzene (123TCB), a non-polar narcotic, and parathion, an acetylcholinesterase (AChE) inhibitor. Hatching and survival after 28 days were significantly reduced in the highest 123TCB treatment (263 μg/l), but not in any of the parathion treatments. Growth of the larvae was negatively affected at parathion concentrations above 20 μg/l, while AChE was only significantly inhibited at the highest concentration, 93 μg/l. No effects on growth were found in the 123TCB treatments. In comparison with acute and chronic studies with both compounds, the ELS test turned out to be less sensitive than chronic studies and more sensitive than acute studies. The difference in sensitivity between the tests systems seems however, to depend on the mode of action of the compound. © 2002 Elsevier Science Ltd. All rights reserved.