In perceptual listening tests, subjects have to listen to short sound examples and rate their sound quality. As these tests can be quite long, a serious and practically relevant question is if participants change their rating behaviour over time, because the prolonged concentration while listening and rating leads to fatigue. This paper presents first results of and hypotheses about changes in the rating behaviour of subjects taking a long-lasting subjective listening test evaluating different algorithms for environmental sound texture synthesis. We found that ratings present small but statistically significant upwards tendency towards the end of the test. We put forward the hypotheses that this effect is due to the accustomation of the subjects to the artefacts present in the test stimuli. We also present the analysis of a second test evaluating wind noises in interor car recordings and find similar effects.