The slides summarize the experience gained from the design, implementation and use of a set of distributed web services for predictive toxicology. The OpenTox framework, collaboratively developed by partners of the EU FP7 OpenTox project www.opentox.org, aims at providing unified access to chemicals, toxicity data and predictive models. The framework is based on two technological decisions: a common information model, based on ontologies, and availability of data and methods via a standardized REST web services interface, where every chemical compound, data set, or processing algorithm has a unique web address, used to retrieve information or initiate the calculations. The challenges of designing a distributed system by a distributed team are discussed. A common application programming interface was defined and implemented independently by several partners. Our open source AMBIT REST software package implements a large subset of the OpenTox API (data and processing), and is available both as online services and as a downloadable archive.