Daytime urinary frequency in children.

linear response to doses ofpropranolol lying within one order of magnitude (say 100-200 mg) using only data for median dose rates, with random individual variation in dose from day to day and standard deviations as wide as those seen in Intersalt. If the relation is logarithmic it is invalid to use the Intersalt data to estimate the effect of a drop of 100 mmol in sodium excretion because 100 mmol would have different effects in different parts of the scale. It seems unlikely that a reduction from 400 to 300 mmol/day in Akita, northern Japan,' would be equivalent to a drop from 130 to 30 mmol/day in another culture. Alternatively, there may be a threshold for effect, not shown in the distribution of salt intake obtained in the Intersalt study. Meanwhile, it is noteworthy that the four communities with low salt intake had a noticeably low prevalence of hypertension. TREVOR C BEARD