Brevetoxins are lipophilic polyether toxins with documented neurotoxiceffects on adult animals. In this study, we extend an earlier study ofciguatoxin (Toxicon 37:1827,1999) to quantify the adverse developmentaleffects of brevetoxins using an exposure paradigm that parallels thematernal-oocyte transfer of toxin. Medakafish (Oryzias latipis) embryos areexposed to brevetoxin six hours post fertilization by microinjection of asmall quantity (2 nanoliters) of brevetoxin (or vehicle) reconstituted in afish oil (triolein) droplet. The brevetoxin-containing droplet is placedadjacent to the larger oil droplet of the yolk sac. Embryos microinjectedwith doses of 0.8 ng/egg (ppm) and higher of brevetoxin-1 exhibit pronouncedcardiovascular (tachycardia) and muscular (hyperkinesis) activity byembryonic day four. Prior to hatching, morphological abnormalities werecommonly found in embryos at the following lowest adverse effect levels: 1.1ppm- lateral curvature of the spinal column; 3.1 ppm- herniation of brainand meninges though defects in the skull; and 3.4 ppm malpositioned eye.Hatching abnormalities are also commonly observed at brevetoxin doses of 2.0ppm and higher with head-first, as opposed to the normal tail-firsthatching. Given the similarity of developmental processes found betweenhigher and lower vertebrates, teratogenic effects of brevetoxins have thepotential to occur among different phylogenetic classes. The observation ofdevelopmental abnormalities following brevetoxin exposure identifies a newspectrum of adverse effects that may be expected to occur following exposureto red tide events.