Using zebrafish embryos and larvae, we examined the temporal patterns of cortisol and expression of genes involved in corticosteroid synthesis and signaling. Embryonic cortisol levels decreased approximately 70% from 1.5 h post-fertilization (hpf) to hatch (~42 hpf), followed by a 27-fold increase in cortisol by 146 hpf. The mRNA abundances of steroidogenic acute regulatory protein, 11-hydroxylase and 11-hydroxysteroid dehydrogenase 2 (11HSD2) increased several fold post-hatch and preceded the rise of cortisol levels. Unlike other teleosts examined to date that possess two glucocorticoid receptors (GR) and one mineralocorticoid receptor (MR), only one GR and MR were identified in zebrafish, which were cloned and sequenced. GR mRNA abundance decreased from 1.5 to 25 hpf, then rebounded and was stable from 49 to 146 hpf. MR transcripts increased continuously from 1.5 hpf and were 52-fold higher by 97 hpf. An acute cortisol response to a stressor was not detected until 97 hpf, while an increase in melanocortin 2 receptor (MC2R) mRNA occurred between 25 and 49 hpf. Collectively, the patterns of cortisol and the expression of cortisol biosynthetic genes and MC2R suggest that the corticoid stress axis in zebrafish is fully developed only post-hatch. The temporal differences in GR, MR and 11HSD2 gene expression leads us to propose a key role for MR signaling by maternal cortisol during embryogenesis, while cortisol secretion post-hatch may be regulating GR expression and signaling in zebrafish.