Studies were carried out to evaluate the influence of variations in sodium balance on the renal response to low-dose infusion of 1-desamino-8- D -arginine vasopressin (dDAVP), and the functional interaction between dDAVP and renal prostanoids. The studies were performed on healthy women in conditions of extracellular fluid volume expansion (SR group, n =9) and depletion (SD2 group, n=6), respectively. The study protocol included hypotonic polyuria (induced by oral water load) and subsequent antidiuresis (induced by low-dose infusion of dDAVP). Three 60-min clearance (cl.) periods were performed during polyuria (cl. P), early (cl. A1) and late (cl. A2) antidiuresis. The urinary concentrations of prostaglandin (PG) E(2) and the stable metabolites of PGI(2) and thromboxane (Tx) A(2), 6-keto-PGF(1alpha) (6KPGF) and TxB(2), were estimated. Paired renal functional explorations were performed in salt retention and salt depletion both in absence and presence of indomethacin (SR.I and SD2.I groups). In both paired and unpaired studies, the early and late effects of dDAVP on the functional excretory variables and the excretion of prostanoids were assessed as percentage variations, (A1-P)% P and (A2-A1)% A1. (I) dDAVP in salt retention and depletion. During early infusion dDAVP produced in both conditions a significant reduction in urinary flow rate, creatinine cl., absolute and fractional excretions of sodium, chloride and potassium; during late infusion dDAVP was effective in inducing a further significant reduction in urinary flow rate. In salt retention compared to depletion the early reductions in sodium and chloride (absolute and fractional) excretions were significantly lower. (II) Indomethacin pretreatment. During early infusion the dDAVP-induced reductions in the urinary flow rate and 6KPGF excretion were enhanced in both conditions. In salt depletion the dDAVP effects in reduction of creatinine cl. and urinary electrolyte excretions were also enhanced. During late infusion the antidiuretic effect of dDAVP was suppressed in salt retention, while in salt depletion creatinine cl., the urinary excretions of electrolytes and both 6KPGF and TxB(2) showed increases significantly different from the dDAVP effects in the absence of indomethacin. In conclusion, (a) the salt-retaining effect of dDAVP was less effective in salt retention compared to depletion. (b) Indomethacin pretreatment affected the renal action of dDAVP in a time-dependent pattern. The early effects in both conditions were consistent with an inhibited synthesis of modulator PGs. On the contrary, the late effects were consistent with the occurrence, at least in salt depletion, of an escape from dDAVP renal action. This escape phenomenon probably depended on a partial regression of the pharmacological inhibition of the modulating PGs.