Components made of short glass fibre reinforced (sgfr) plastics are, due to enhanced design possibilities and total cost reduction, increasingly used in under-the-hood applications in the automotive industry. During operation, these parts are often exposed to cyclic loading in combination with aggressive environmental media such as engine coolant. As a result of the hygroscopic behaviour of some polymers, the exposion to this type of media leads to moisture absorption and therefore to significant changes in weight and dimensions of the material, often referred to as swelling. The processes of ageing, such as reduction of fibre-matrix adhesion, as well as swelling of the polymer typically lead to a change of anisotropic mechanical properties which has to be taken into consideration in designing the product. The objective of this thesis is to characterise the effects of moisture absorption, anisotropic swelling and ageing on the mechanical properties of a sgfr polyamide with 50 wt% glass fibres. The influence of aggressive environmental media is examined by quasi-static tensile tests as well as cyclic fatigue tests. For this purpose, specimens milled out from injection moulded plates are stored in engine coolant and water up to 1500 hours. In addition to the experimental part of the thesis, a simulation chain for the evaluation of occurring stresses and strains due to swelling is developed. This numerical tool supports the simulation of the diffusion process of water absorption and the resulting distribution of stresses. This information can subsequently be used in distortion analysis of complex parts. The results show a water absorption up to 4.1 % of the specimen weight which leads to a decrease in tensile strength of up to 55 %. A reduction of up to 85 % was determined on specimens stored in engine coolant at elevated temperatures. The cyclic tests showed a significant media-independent decrease in fatigue strength of up to 82 % after conditioning in water as well as coolant.