Experimental model systems and bioassays are used in ecotoxicology and environmental toxicology to provide information for risk assessment evaluation of new chemicals and to investigate their effects and mechanisms of action; in addition, ecotoxicological models are used for the detection, control and monitoring of the presence of pollutants in the environment.

  Most of the procedures used in regulatory and non-regulatory toxicology are carried out on mammals, but public pressure to minimize the use of vertebrates in ecotoxicology testing and scientific interest in promoting the study of the effects of chemicals on both terrestrial organisms and aquatic environment is growing. The use of invertebrates organisms, microorganisms, and plants, despite their being far more abundant in nature than vertebrates, however, is still not well represented in ecotoxicology.

  The most promising alternatives in environmental toxicology encompass the use of fewer organisms with limited sensitivity and/or which are non-protected by legislation controlling animal experiments. These include bacteria, fungi, algae, plants, invertebrates and vertebrates used at early stages of development, from fish, amphibians, reptiles and birds to mammals. Also the employment of in vitro methods using material from these organisms is of particular interest.

  The effects of the insecticide pentachlorophenol were studied using a battery of ecotoxicological model systems, including inhibition of bioluminiscence in the marine bacterium Vibrio fischeri, growth inhibition of the alga Chlorella vulgaris, micronuclei induction in the plant Allium cepa, the inhibition of MTT reduction in Vero cells, the inhibition of cell growth in the salmonid fish cell line RTG-2, derived from the gonad of rainbow trout (Oncorhynchus mykiss), and the inmobilization of the cladoceran Daphnia magna. This uncoupler of oxidative phosphorilation was very toxic for all the model systems. The most sensitive system was the induction of micronuclei in A. cepa, followed by bioluminiscence inhibition on V. fischeri at 60 min. Daphnia magna and C. vulgaris showed intermediate sensitivity. The less sensitive systems were MTT reduction on Vero monkey cells at 24 hr and inhibition of cell proliferation on the RTG-2 cells.

  The complexity of the obtained results, with very different effects according to the test system and exposure period employed, shows that a single bioassay will never provide a full picture of the quality of the environment. A battery of test systems and indicators would be representative of a wide range of organisms. For the time being, a minimum ecotoxicological in vitro test battery should at least include bacteria, algae, invertebrates and fish cell lines.

  Many opportunities to change the actual scene will be implemented in the near future. More efficient risk assessment procedures should be designed, integrating more flexible testing methods into tier testing schemes that employ only the neccesary tools for each case. A careful selection of the most appropiate species, strains and development stages for testing can be combined with a reduction in the number of required assays and/or animals/assay. The development of mechanistic assays, based on relevant new endpoints for mammals, invertebrates (bacteria, fungi, algae, protozoans) and cell cultures, may be crucial to this process.

  The regulatory acceptance of new ecotoxicological methods has been very slow, particularly for the evaluation of new chemicals. The D. magna inmobilization assay and the algae growth are included in most environmental legislations, including OECD Guidelines for testing of Chemicals. However, the V. fischeri test has only been adopted by some regulations for the characterization of hazardous waste. Some alternative procedures (mainly in vitro) has been applied and integrated as useful tools in environmental contamination and toxicology and are now routinely used in areas such as pollutant monitoring and control by, both scientist and the industry. Their continuing validation and acceptance as real alternatives to animal testing, and particularly to replace fish studies, should be promoted.