In conscious rats prostaglandin E2‐ and F2aexcretion rates increased from about 20 to more than 100 pg/min/g kidney weight as urine flow decreased from about 10 to below 1 μ/min/g kidney weight following 24 h of water deprivation confirming previous data from surgically prepared rats (Leyssac&Christensen 1981b). There was no difference in this response between rats on a normal sodium diet and severely sodium depleted rats. Additional supply of polyunsaturated fatty acids in the food did not influence the results. Elevated levels of prostaglandin excretion did not correlate with urine osmolality. The reliability of the radioimmunoassays used was documented by comparison with measurements by gas chromatography‐mass spectrometry. The analysis showed that these two methods provide almost identical estimates of urinary PGE2and PGF2acontent. It is concluded that renal prostaglandin production is activated by a negative feedback mechanism as tubular and/or urine flow rate decreases towards subnormal va