The influence of the oestrous cycle on spontaneous and dipsogen-induced drinking was studied in female rats. Spontaneous fluid intake was lowest on the day of oestrus. Drinking induced by subcutaneous isoprenaline, and by angiotensin II (injected into the preoptic area), also showed marked cyclical variation, being lower at pro-oestrus and oestrus than at other stages of the cycle. Drinking induced by subcutaneous hypertonic NaCl or by intracranial carbachol did not vary with the oestrous cycle. Cyclicity of spontaneous and of angiotensin-induced water intake was not apparent in rats before puberty or after ovariectomy. Ovariectomy reduced drinking in response to isoprenaline. Treatment with oestradiol benzoate (20 micrograms) caused a reduction in spontaneous water intake, but a marked increase in the drinking response to isoprenaline. Treatment with oestradiol benzoate and progesterone (2.5 mg) caused a larger decrease in spontaneous water intake and an insignificant increase in isoprenaline-induced drinking. Water intake induced by subcutaneous hypertonic saline was unaffected by gonadal steroids. The results provide further evidence for the view that the thirst of extracellular origin, in which the renin-angiotensin system is involved, is brought about by mechanisms different from those that respond to cellular dehydration. Only drinking caused by activation of extracellular mechanisms appeared to be sensitive to the ovarian cycle and to ovarian hormones.