The composition of amniotic fluid was found to vary in dependence on storage conditions. Catalytic concentrations of creatine kinase and lactate dehydrogenase were considerably decreased after storage at -20°C, and the decreases were less marked after storage at -70°C. Other analytes were more stable. Consequently, the optimal storage conditions for amniotic fluid samples may differ for various analytes. The low inter-individual variability of some analytes (Na, K, Ca, Cl) suggests that their concentrations are homeostatically controlled during the gestational period studied. In contrast, the high variability of enzyme activities may be due to more rapid quantitative changes during normal gestation; in addition, enzyme activities may be affected by certain pathological and non-pathological conditions of the fetus.