Acute swim stress (3 min at 32 degrees C) in mice produces increases in the binding of MK-801 to the NMDA subclass of glutamate receptors to forebrain membranes prepared from male mice. Scatchard analyses indicate that the observed increases in the binding of MK-801 in membranes from male mice are the result of changes in the affinity and density of low-affinity binding sites and in the density of high-affinity binding sites. In female mice, any changes in the binding of MK-801 appear to be much less pronounced and restricted to the low-affinity binding sites. These results are in contrast to the situation with binding to GABA receptors where acute swim stress increases GABA binding in forebrain membranes much more in female than in male mice. This indicates significant sex differences in the responses of receptors for the major excitatory and inhibitory transmitters to acute swim stress. These rapid changes in MK-801 binding may result from changes in endogenous modulators as appears to be the case in the acute swim stress-induced changes in GABA binding. As with GABA binding, the endogenous modulators are likely to include steroids, the sex differences reflecting differences in modulation by gonadal steroids and the stress-induced changes reflecting differences in modulation by adrenal steroids. Estradiol, progesterone, and corticosterone treatments have been reported by other workers to influence the properties of glutamate receptors.