Meta-analysis has found that early life-cycle portions of full life-cycle tests usually estimate an MATC within a factor of 2 of full life-cycle estimates in saltwater and freshwater fish. In 83% of 72 tests, the ELS portion resulted in the same MATC as the full life-cycle estimate, and the remaining 17% were within a factor of 2. There remain some limitations with early life stage toxicity testing. Although ELS tests are quicker and more cost-efficient than full life cycle tests, they remain resource- and time-intensive. Fish early life stage (FELS) tests require hundreds of fish and 1 to 5 months to complete. Other issues include the lack of mechanistic information, differing sensitivities between species, and insensitivity to parental exposure. ELS tests don't provide information on the toxicant's mechanism of action. Sensitivity to specific toxicants varies with species, so the most sensitive or most important species should be tested in each case. ELS tests appear to be insensitive to parental exposure, and MATCs are generally the same for embryos of both exposed and unexposed parents. This could be due to the mode of action of the toxicant or the variability and insensitivity of ELS test design. Additionally, growth response has been found to be an insensitive endpoint in ELS tests with fish, having little bearing on the estimation of an MATC. Growth response could be omitted to reduce the duration and cost of screening tests. In a typical early life stage toxicity test, a flow-through dilutor system administers different concentrations of a toxicant to different test chambers. At least five different concentrations of a toxicant are tested, plus controls, with at least two exposure chambers for each treatment. The length of the exposure depends on the test species. For example, fathead minnow tests are 1–2 months long, while brook trout tests are around 5 months long. Growth and survival are the typical endpoints, for which an MATC can be found. Standard methods for ELS tests have been established by the OECD, ASTM International, the EPA, and Environment Canada. An extended ELS test has been examined as a potential surrogate for a fish full life-cycle test to detect weak environmental estrogens. Endocrine active chemicals (EACs) are ubiquitous in the environment, prompting the need for better screening assays to predict their effects, especially in aquatic species. Slightly longer ELS tests could be used instead of full life-cycle tests, taking into account sensitive windows of exposure like sexual differentiation and early gonadal development.