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1. The time course and extent of changes in urinary flow and in the outputs of urea, Na⁺, K⁺, and NH₄⁺ over a period of 7 hr in conscious rats during water and osmotic (mannitol) diuresis were determined, and compared with spontaneous changes in non-diuretic animals.

2. In non-diuretic rats, a morning rise and subsequent decline in urinary osmolal, sodium, potassium and ammonium outputs occurred, possibly attributable to circadian rhythms.

3. Water diuresis was accompanied by (i) a rapid increase in urea excretion during the phase of increasing urine flow, followed by a fall in later periods to values similar to those in non-diuresis, (ii) a slower increase in sodium output, continuing after the establishment of the constant water load, (iii) unchanged potassium excretion, but slightly increased ammonium outputs.

4. Mannitol diuresis was accompanied by (i) a rapid increase in urea outputs which subsequently fell but remained significantly higher, (ii) a steep rise in sodium and potassium outputs to values which remained far higher than those in non-diuretic and water diuretic animals.

5. The changes in mannitol diuresis are considered to result mainly from decreased tubular reabsorption, due to the lowered intraluminal sodium, potassium and urea concentrations and increased intratubular fluid flow. Some of the acute increase in urea excretion may be due to washout of medullary urea into the tubular fluid.

6. In water diuresis, some of the changes in solute excretion may similarly result from altered tubular reabsorption, perhaps influenced by suppression of anti-diuretic hormone (A.D.H.). In addition, the slower changes in sodium output may be related to several consequences of change in body fluid volume.

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