If exploration of outer space is going to be a major human enterprise in the future, it is important to establish the nature of the biological response to the space environment. In one of the recent Soyuz missions to serve the ISS, the Spanish Soyuz Mission in October 2003, we sent a group of Drosophila pupae that underwent almost complete development there. Microarray analyses of the RNAs extracted from flies fixed in the ISS revealed that a relatively large set of genes (15% of the total number assayed) suffered a significant expression change in these conditions. Furthermore, the samples had to be transported to the launch site and it was necessary to slow down their development by exposing them to a lower temperature, fully compatible with pupal development. Such a pre- exposure had an effect by itself on the pattern of gene expression observed after pupal development at normal temperature, but the two environmental factors seemed to act synergistically together with the containment in the type I container. These findings indicate the importance of maintaining a vigorous scientific program in the ISS to understand the consequences of the modified environment in outer space on living organisms.