The effects and mechanisms of action of diepoxybutane (DEB) and mitomycin C (MMC) were investigated on sea urchin embryogenesis, (Sphaerechinus granularis and Paracentrotus lividus). DEB- and MMC-induced toxicity was evaluated by means of selected end-points, including developmental defects, cytogenetic abnormalities and alterations in the redox status [oxygen-dependent toxicity, Mn-superoxide dismutase (MnSOD) and catalase activities and glutathione (GSH) levels]. Both DEB and MMC exhibited developmental toxicity (at concentrations ranging from 3 × 10–5 to 3 × 10–4 M and 3 × 10–6 to 3 × 10–5 M, respectively) expressed as larval abnormalities, developmental arrest and mortality. The developmental effects of both compounds were significantly affected by oxygen at levels ranging from 5 to 40%. These results confirmed previous evidence oxygen-dependent MMC toxicity and are the first report of oxygen dependence for DEB toxicity. Both DEB and MMC exerted significant cytogenetic abnormalities, including mitotoxicity and mitotic aberrations, but with different trends between the two chemicals, at the same concentrations as exerted developmental toxicity. The formation of reactive oxygen species was evaluated using: (i) luminol-dependent chemiluminescence (LDCL); (ii) reactions of the main antioxidant systems, such as GSH content and MnSOD and catalase activities. The results point to clear-cut differences in the effects induced by DEB and MMC. Thus, DEB suppressed GSH content within the concentration range 10–7–3 × 10–5 M. The activity of catalase was stimulated at lower DEB levels (10–7–10–6 M) and then decreased at higher DEB concentrations (≥10–5 M). Increasing MMC concentrations induced LDCL and MnSOD activity (≥10–6 M) greatly and modulated catalase activity (10–7 – 10–6 M). GSH levels were unaffected by MMC. The results suggest that oxidative stress contributes to the developmental and genotoxic effects of both toxins studied, although through different mechanisms.