1. In the brain, glucocorticoids bind to both the glucocorticoid receptor (GR) and the mineralocorticoid receptor (MR). These receptors show clearly distinct developmental patterns in the infant rat. 2. Low levels of GRs are present around the time of birth throughout the brain. Concentrations rise slowly, and do not achieve adult levels until the third week of life, approximately. GR affinity for corticosterone is higher perinatally than at later ages. Receptor microdistribution changes dramatically during ontogeny. In particular, certain regions, such as the suprachiasmatic nucleus of the hypothalamus, express high levels of receptor only during the first week of life. GRs may show impaired capacity to undergo transformation and/or nuclear translocation during the second postnatal week. Environmental manipulations during early ontogeny (e.g., early handling) may have permanent effects on GR capacity. 3. MRs are present at very low concentrations in the first days of life. Binding capacity rises rapidly thereafter and resembles that found in the adult by the end of 1 week. Neither binding affinity in vitro nor overall distribution changes with age. As in the adult, low doses of corticosterone, in vivo, bind mainly to the MRs. Levels of corticosterone are low and relatively unperturbable in the intact infant rat. It is likely, therefore, that most of the physiological actions of this hormone during this period are mediated by the MR.