AbstractGold(I)-containing complexes are used in drug discovery research for rheumatoid arthritis, cancer, and parasitic infections. In this study, we tested the bioactivity of gold(I) complexes in vivo using planarians. The planarian Schmidtea mediterranea possesses orthologues of tumor suppressor genes, such as p53, that, when silenced, cause deregulation of cell proliferation and apoptosis. In this context, we tested two triethylphosphine-gold(I) complexes (AdO and AdT) to determine if they can attenuate phenotypes that result from p53 inhibition. First, we identified the drug concentration that did not affect survival or regeneration and evaluated the drug’s effect on cell division and apoptosis. We found that AdT treatment decreased the number of mitotic cells and that all drug treatments increased the number of apoptotic cells. We then performed p53(RNAi) and drug treatments concomitantly and observed the phenotype progression. Drug treatment increased survival three-fold and decreased apoptosis, which resulted in an attenuated phenotype. Our results indicate that planarians can be treated with gold(I) complexes, and that this treatment can diminish the p53(RNAi) phenotype and extend survival. In this work we show that planarians can be used as a model to study the in vivo effect of gold(I) complexes and to further investigate their mechanisms of action.