(GE/m**3). According to this definition one odour unit is the amount of odorants in one cubic meter of air at odour thres­ hold level. The new definition is a real concentration and gives a better form of input parameter for dispersion models. On the basis of guideline VDI 3881 parts 1, 2, and 3 ringtests were carried out with different odorants. The results can be summerized as follows: -The dispersion of results varies and depends on the compo­ sition of the participants and on problems of sampling and preparation of odorous sample. Lower dispersion is obtained when results with obvious errors in application of guide­ lines or with large deviations from mean value are excluded. -Participants of the Netherlands get systematically lower threshold values than the others. The reason has to be investigated. -All findings of the ringtests lead to the conclusion that it is possible to determine odour thresholds which do not differ by more than factor 10. At present another ringtest is in preparation. This test will be carried out in summer 1985. The French collegues will also participate in this test. Experience of all ringtests will be reported in part 4 of guideline VDI 3881. Guideline VDI 3882 deals with the determination of odour intensity and hedonic tone. The members of the working group "odorous substances" assume that odour threshold and odour concentration are insufficient for the characterization of odorous perception. They recommend to judge the odour inten­ sity and the hedonic tone by category estimation. Moreover, it is their opinion that the odour determination with olfacto­ meters is not suitable to assess odour in ambient air. There­ fore they are preparing two guidelines dealing with these problems. Guideline VDI 3883 gives instructions on the regis­ tration of nuisance by interviews with nearby residents of emitting plants or inhabitants of industrial areas. Addition­ ally guideline VDI 3940 describes the determination of odour in ambient air by inspection panels based on the following idea: During constant conditions as to the class of weather, wind speed, and wind direction each local point is charac­ terized by a frequency of odour perception representing the probability to perceive an odour. The situation at a local point will be have to determine the portion of a year with a frequency of odorous perception greater than 5 % in a random test. Both guidelines, VDI 3882 and 3940, should give corres­ ponding results. Guideline VDI 3781 part 5 completes the complex of odour determination and judgement with the calculation of disper­ sion models. The calculation methode and odour determination by panelists should give comparable results. The following summery can be given. Odour measurements with olfactometers is only a small part of the whole field of odour determination in ambient air and the measurement of odour nuisance must be approached in the near future with appropriate urgency.