A study of the effect of in-plant recycling on the durability of glass fiber reinforced polyamide 66 is reported. Injection-molded test bars were exposed to thermal aging, coolant aging, and creep testing. It was shown that thermal aging and coolant aging induced similar effects. The onset of embrittlement was revealed by both a reduction in elongation at break and loss of impact resistance. Samples based on 100% in-plant regrind exhibited a more intense embrittlement compared to virgin samples during both thermal and coolant aging. For samples based on 25% recycled material, the deterioration rate was similar to that of virgin samples during thermal aging but slightly faster during coolant aging. Creep experiments indicated that in-plant recycled materials can display reduced dimensional stability, which may be explained by a reduced degree of reinforcement due to fiber breakage during the recycling operation. Knock-down (KD) factors were defined and determined for the in-plant recycled samples. For creep resistance, tensile strength and modulus, the KD factors were strongly affected by fiber length. The highest ICD factor, as determined for samples based on 25% and 100% recycled material, was 8% and 16% respectively.