The achievement of successful pregnancies and births after in-vitro fertilization and embryo transfer of frozen-thawed human oocytes has stimulated further work on improving the established methodology. The present investigation was conducted on 1837 mouse oocytes, 1785 mouse pronuclear stage embryos, 1400 hamster oocytes and 1024 hamster pronuclear-stage embryos. In an effort to study the advantages and disadvantages of a newly introduced, 1,2-propanediol (1,2-PROH)-based system over the conventionally used dimethylsulphoxide (DMSO)-based methodology, a direct, prospective comparison between the two cryoprotectants was undertaken in a randomized trial. The combination of 1,2-PROH and DMSO potentiates their cryoprotective effect on mouse and hamster eggs. 1,2-PROH seems to improve cryopreservation in the animal system, though not significantly. Four different protocols were used to evaluate the effects of two changing experimental parameters. These were, firstly, the intermediary temperature attained before placing the cells into liquid nitrogen and, secondly, the modification of the method of adding and removing the cryoprotectant. The morphological survival rate, fertilization rate and developmental rate were significantly better in the low intermediary temperature compared with the high intermediary temperature system of cryopreservation. In addition, the rate of zona pellucida breakdown diminishes considerably in the former compared with the latter system. The 'rapid' sucrose method of cryoprotectant equilibrium and removal showed, in most groups, results which were lower than, or equal to, the traditionally used 'multiple-step', 'slow' method.

Potential hazards of environmental chemicals are a function of both the toxicity and exposure. Currently, knowledge of toxicity and exposure of pollutants is mainly based on data which were obtained by differentiated toxicological tests as well as environmental monitoring analysis. On the other hand, it has been recognized that biological activity/toxicity and environmental fate of chemicals are related to molecular structures and often can be predicted on the basis of quantitative structureactivity relationship. The aim of this paper is to discuss an approach for assessing acute toxicity of environmental chemicals using molecular structure parameters in order to set priorities for toxicological testing.

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Chemical substances can be transformed under the influence of physical/chemical and/or biochemical active factors. By this, the ecotoxicological and hygienic‐toxicological properties and effects of the substances are changed also with respect to toxicity, late effects (carcinogenetic, mutagenic, teratogenetic ones) as well as the bio‐ and geo‐accumulation behaviour. Such transformations occur also during water treatment, and therefore also the properties and effects of the transformation products have to be taken into account for determining the limit values for pollutants in raw water for the production of drinking water. Since frequently we have incompletely known transformation products or mixtures of substances, a coefficient of hazard is suggested for the evaluation. It is the quotient of the threshold concentrations of the initial substance and of the transformation products for the different (organoleptic, toxic, carcinogenic, teratogenetic etc.) effects. For several organic substances it is demonstrated that the transformation products can be regarded less toxic compared with the initial substance. Strong oxidants, however, cause an increased danger in many cases.