The effect of doxorubicin and phenytoin on Paracentrotus lividus development and fertilization is reported. Doxorubicin at concentrations between 1.4 and 1.8 microM causes severe disturbances of development and at 2.0 microM, complete embryonic degeneration, if added in the period from fertilization on. Concentrations up to 20 microM are ineffective on development if the treatment is started at 12 h after fertilization. Phenytoin at concentrations between 4.0 and 7.0 microM causes severe disturbances of development, if added from 5 min after fertilization on. Concentrations of 10 microM are lethal to all embryos. Again, if the drug is added at 12 h after fertilization, no effect is observed up to the concentration of 20 microM. Both drugs are ineffective on the fertilization process up to the concentration of 20 microM for doxorubicin and of 200 microM for phenytoin.