Summary Equine rhabdomyolysis syndrome (ERS) has been reported to occur more frequently around oestrus. Because electrolyte imbalances have been implicated in pathogenesis, this preliminary study was designed to investigate the possible influence of concentrations of reproductive hormones, progesterone and oestradiol‐17β upon the 24 h%FE values of certain electrolytes. Over a 35 day period, daily plasma progesterone concentrations were measured in 4 mature Thoroughbred mares and serum oestradiol‐17β concentrations in 3 of the mares. At various times during oestrus and dioestrus, 24 h urine collections were carried out and mean 24 h %FE values were determined, in each individual, for sodium (Na), potassium (K), chloride (CI), calcium (Ca), phosphate (PO4) and magnesium (Mg). Although the mares were maintained on a constant dietary intake of electrolytes, group mean %FE values showed cyclical trends which appeared to fluctuate in relation to the stage of the cycle. Large and relatively sudden increases in the volumetric urinary excretion of Na, Ca and Mg (mmol/24 h) were seen in samples collected during the period from 2 days preceding to 4 days following oestrus, compared to the rest of the cycle. Significant correlations were found, in one or more individual animals, between progesterone concentrations and 24 h %FE values for Na, K, Cl, Ca and PO4 and between serum oestradiol‐17β concentrations and Ca, Mg and Cl. This suggests that ovarian steroid hormones may exert an influence on urinary excretion of electrolytes in the mare. Such an influencecould be correlated, in certain individuals, to a significant effect on electrolyte status which, in turn, could potentially result in an increased risk of ERS at particular times of the oestrous cycle.