The urinary excretion of free dopamine, norepinephrine, and epinephrine could reflect the contribution of the neural release and filtration of these catecholamines as well as the intrarenal tubular synthesis and metabolism of dopamine. Because these catecholamines are rapidly metabolized, the excretion of the free amines represents only a fraction of the total release and synthesis by the kidney. The present study determined the effect of increasing dietary phosphate intake on the excretion of free dopamine, norepinephrine, and epinephrine and their primary stable metabolites. Seven male rats were placed in metabolic balance cages and fed 12 gm/day of normal phosphate diet (NPD) (0.7% inorganic phosphorus [Pi]) for 4 days and then fed a high phosphate diet (HPD) (1.8% Pi) for 4 days. Twenty-four-hour urine samples were collected for determination of free catecholamines, their major stable metabolites, and electrolyte excretions. The urinary excretion data for the seven rats was combined for all 4 days of each dietary regimen. Increasing phosphate intake from 0.7% to 1.8% significantly increased free dopamine excretion by 23%, from 5.6 +/- 0.2 to 6.8 +/- 0.1 micrograms/day (n = 7, p < 0.05). This increase in free dopamine excretion was associated with similar increases in urinary excretion of dopamine glucuronide, 21.6 +/- 1.3 to 27.9 +/- 1.8 micrograms/day (32%) and the dopamine metabolite DOPAC, 9.4 +/- 0.5 to 12.1 +/- 0.6 micrograms/day (30%) and total dopamine excretion from 32.9 +/- 1.7 to 41.0 +/- 1.9 micrograms/day (27%). Plasma DOPA levels were unchanged by increased dietary phosphate intake; however, plasma norepinephrine levels decreased significantly. Excretion of free or sulfated norepinephrine was not changed by increased phosphate intake. However, excretion of MHPG, a metabolite of norepinephrine and epinephrine, decreased significantly, from 33.7 +/- 2.1 to 23.9 +/- 0.8 micrograms/day, n = 7, p < 0.05.(ABSTRACT TRUNCATED AT 250 WORDS)