Rats were depleted of vitamin B6 by treatment with isoniazid, and then maintained on diets providing: no vitamin B6, an adequate amount (1.2 mg/kg diet) or a very large amount (120 mg/kg) for 5 weeks. The uptake of a tracer dose of [3H]-oestradiol into the nuclei of liver and uterus was significantly greater in deficient animals than in those receiving an adequate or greater than adequate amount of the vitamin. Similarly the accumulation of oestradiol in the region of the brain corresponding to the hypothalamus, pre-optic area and septum (the major oestradiol-sensitive regions of the central nervous system) was higher in deficient animals than in other groups. There were significant inverse correlations between the uptake of oestradiol into target tissues and vitamin B6 nutritional status as determined by the concentrations of pyridoxal phosphate in plasma and liver. Uteri from deficient animals were significantly smaller than those from animals receiving an adequate or greater amount of vitamin B6, and the induction of uterine peroxidase by oestradiol was impaired. It therefore seems likely that despite the greater net accumulation of steroid in target tissues, vitamin B6 deficiency impairs biological responsiveness to oestrogens. It is suggested that this may be due to a failure of the recycling of oestradiol receptors from the nucleus.