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Steroids are lipophilic hormones that impinge upon virtually every physiological process in vertebrates. The synthesis of the correct type and amount of steroid at the proper time requires the precise regulation of the steroidogenic acute regulatory protein (STAR) and various steroidogenic cytochrome P450 and hydroxysteroid dehydrogenase enzymes. The transcription factors steroidogenic factor-1, DAX-1 and chicken ovalbumin upstream promoter transcription factor are critical to the coordinated expression of steroidogenic genes. Factors that alter the activity of steroidogenic proteins or the expression of the genes encoding them can therefore have dramatic effects on the health and reproductive success of vertebrates. We have isolated cDNAs encoding STAR, steroidogenic enzymes and steroidogenic transcription factors from a teleost, the Atlantic croaker (Micropogonias undulatus) in order to investigate the effects of natural and anthropogenic stressors on gonadal steroidogenesis. The Atlantic croaker is an abundant and ecologically important species for which a wealth of ecological and physiological data exists, making it an excellent model for the study of environmental stress and its effects on reproduction. Atlantic croaker steroidogenic cDNAs encode proteins that are 28-86% identical to corresponding proteins in other species. Using RT-PCR, we detected steroidogenic mRNAs in ovary, testis and head kidney (adrenal cortex homolog). Preliminary evidence indicates that both reproductive hormones (e.g. gonadotropins) and stress hormones (e.g. adrenocorticotropic hormone, cortisol) can regulate gonadal steroidogenesis by modulating the expression and activity of steroidogenic proteins.