Abstract: Transfer RNA molecules are involved in a variety of biological processes, implying complex recognition events with proteins and other RNAs. From a structural point of view, tRNAs constitute a reference system for studying RNA folding and architecture. A deeper understanding of their structural and functional properties will derive from our ability to model accurately their dynamical behavior. We present the first dynamical model of a fully neutralized and solvated tRNA molecule over a 500-ps time scale. Starting from the crystallographic structure of yeast tRNA(Asp), the 75-nucleotide molecule was modeled with 8055 water molecules and 74 NH4+ counterions, using the AMBER4.1 program and the particle mesh Ewald (PME) method for the treatment of long-range electrostatic interactions. The calculations led to a dynamically stable model of the tRNA molecule. During the simulation, all secondary and tertiary base pairs are maintained while a certain lability of base triples in the tRNA core is observed. This lability was interpreted as resulting from intrinsic factors associated with the "weaker" hydrogen bonding patterns seen in these base triples and from an altered ionic environment of the tRNA molecule. Calculated thermal factors are used to compare the dynamics of the tRNA in solution and in the crystal. The present molecular dynamics simulation of a complex and highly charged nucleic acid molecule attests to the fact that simulation methods are now able to investigate not only the dynamics of proteins, but also that of large RNA molecules. Thus they also provide a basis for further investigations on the structural and functional effects of chemical and posttranscriptionally modified nucleotides as well as on ionic environmental effects.