The role of the skeleton in electrolyte equilibrium, well known for various diseases, remains difficult to understand during chronic hypercapnia. An experimental study of normoxic (O2:21%) hypercapnia (CO2:8 +/- 1%) was carried out for two, four and six weeks, followed by a systematic quantitative determination, in thigh-bone samples of Na+, K+, Ca++, PO4--, N2 and CO2 in 72 rats, and of total H2O and extracellular H2O (H2Oe) in 129 rats. Considering the mean values of groups (from 16 to 42 subjects for each group), at various times of hypercapnia, bone K+ was increased during hypercapnia (+3 to 4 X 10(-3) mEq/g fresh tissue), Ca++ diminished (--12.5 to 15.4 mEq). PO4-- and Na+ temporarily decreased at two and four weeks of hypercapnia. On account of the scatter of individual results, only the variation of K+ was statistically significant (at two weeks). This increase in bone K+, accompanying a partially compensated acidaemia, is to compare with the significant hyperkaliemia observed at two and four weeks, whereas this period is characterized by a decrease in K+ in skeletal muscle, as shown in a previous work. In a group of 72 rats, the analysis of correspondances and correlations points out the bone CO2 as a very significant variable, opposite to the variable H2Oe. PO4--is positively correlated to Na+. The complexity of the results does not permit a decisive interpretation of the phenomenon. On the other hand, this study corroborates the bone calcium loss and reveals the gain in bone potassium during hypercapnia.