Electrophysiological field potentials recorded from in vitro hippocampal slice preparations show responses to exogenous gonadal steroids added to the incubation medium. The peak effect of the addition of 17-beta-estradiol occurred at a 100 pmol concentration; the CA1 field potential was increased by an average of 148 percent. 17-alpha-estradiol, often used as a negative control in experiments demonstrating estrogen specificity of receptor binding sites and biological responses, had no effect on field potentials following addition of drug to the incubation medium. The addition of a 100 pmol concentration of 17-beta-estradiol to the same slices which had been pretreated with 17-alpha-estradiol, blocked the facilitatory response elicited by the 17-beta-estradiol administered alone. Since no enhancement of the field potential is observed with 17-beta-estradiol following pretreatment of 17-alpha-estradiol, this would support the hypothesis that hippocampal modulation by gonadal steroid hormones may be due to involvement of an estrogen receptor mediated phenomena.