Abstract Steroid hormone actions on gene expression in the brain provide a vast array of examples of environmentally-controlled gene expression. Recent studies of steroid hormone receptors and the effects they mediate have led to new findings which change our notions about the type and speed of plastic changes which the brain is capable of undergoing. We have found that estradiol and progesterone induce dendritic spines and new synapses as well as regulate oxytocin receptors in hypothalamus, whereas adrenal steroids affect neuronal survival and dendrite structure in the hippocampus as well as regulate several neurochemical processes throughout the brain related to the diurnal rhythm and adaptation to stress. Studies of estrogen and progesterone action in the hypothalamus have also shed some light on the nature of developmentally programmed sex differences in the brain. Moreover, recent work indicates that progesterone is a versatile steroid which not only affects gene expression but also produces effects via membrane receptors which alter release of neuroactive substances, chloride flux and the distribution of oxytocin receptors of the hypothalamus.