1. The role of the median raphe nucleus (MRN) and of increased central serotonin (5HT) synthesis/release in the mediation of Na+ excretion (UNaV) and K+ excretion (UKV) and of urine output (UV) was evaluated for 120 min. 2. Male Wistar rats weighing 220-280 g were used in each group of 12-13 animals. The rats implanted with a cannula in the MRN were injected with saline (0.5 microliters) or with 5.0 and 15.0 ng/0.5 microliters kainic acid (KA), an excitatory amino acid (EAA). Another group of rats was injected ip with 200 mg/kg saline or tryptophan, the initial precursor of 5HT synthesis. 3. Injection of both kainic acid and tryptophan led to increased Na+ excretion, but the magnitude and time course were different for each treatment. 4. Both KA doses were effective in increasing UNaV (0.61 +/- 0.08, mean +/- SEM, and 0.95 +/- 0.19 microEq/min, respectively, vs 0.27 +/- 0.04 microEq/min for saline at 60 min). The effect on UKV was statistically significant with the 15.0 ng dose (0.44 +/- 0.05 microEq/min vs 0.25 +/- 0.03 microEq/min for saline) at 20 min. 5. Tryptophan administration caused an initial gradual increase in UNaV which became steady and significant after 60 min (1.02 +/- 0.15 microEq/min vs 0.36 +/- 0.06 microEq/min for saline), as well as an increase in UKV (0.58 +/- 0.06 microEq/min vs 0.26 +/- 0.04 microEq/min for saline) at 60 min and throughout the remainder of the observation period. 6. KA-induced MRN stimulation and systemic tryptophan overload significantly increased UV at 60, 80 and 100 min (30 to 97% above control values).(ABSTRACT TRUNCATED AT 250 WORDS)