In addition to nanoparticles, high concentrations of pharmaceuticals such as indomethacin have also been detected in wastewater streams. However, there is a lack of data on the potential interactions of nanoparticles and pharmaceuticals and their potential effects on aquatic organisms. Therefore, in this paper we investigated the influence of silver nanoparticles (5 and 50 μg L -1) and indomethacin (0.001 – 10 mg L -1), individually and as combinations, on embryo development of the sea urchin Arbacia lixula and if these influence reactive oxygen species generation in larvae. At indomethacin concentrations of 0.1, 1 and 10 mg L-1, the number of normally developed embryos decreased. LD50 was calculated at 0.11 mg L -1. At concentrations from 1 mg L -1 larval development was halted. For embryos treated with indomethacin (0.001 – 0.01 mg L -1) in combination with silver nanoparticles (50 μg L-1), larvae reached the same developmental stage as controls. However, low concentrations of silver nanoparticles (5 μg L-1) were found to increase the toxicity of indomethacin, with all larvae undeveloped at an indomethacin concentration of 0.01 mg L -1. The cause of such antagonistic or synergistic effects may be related to the interaction of indomethacin with silver ions or the nanoparticles' surfaces, however more research is required to uncover the role each plays. Reactive oxygen species concentrations were low for most combinations of indomethacin and silver nanoparticles, suggesting that indomethacin may play an anti-inflammatory role in ameliorating the toxic effects of the silver ions or nanoparticles.