To investigate the possibility that a proliferative non-neoplastic process influences extracellular cyclic nucleotide concentrations, we measured plasma cyclic AMP and cyclic GMP levels in 38 patients with homozygous beta-thalassaemia. This group consisted of 20 patients with thalassaemia major transfused regularly (mean pre transfusion Hb levels, 11 g/dl), and 18 patients with thalassaemia intermedia who did not require regular blood transfusion (mean Hb levels, 8.7 g/dl). In the patient group, plasma cyclic AMP levels were similar to those of 37 normal subjects matched for age and sex, whereas plasma cyclic GMP levels were markedly higher. Moreover, in the thalassaemic patients there was a significant negative correlation between plasma cyclic GMP levels and haemoglobin concentrations, suggesting that their marked erythroid hyperplasia may play a role in determining alterations in extracellular cyclic GMP levels.