Abstract. The effect of dopamine infusion (4 μg · kg−1 · min−1 from 9.00 to 13.00 h) on serum LH and FSH concentrations were studied in 15 patients (10 women, 5 men) with diffuse toxic goitre, and 10 healthy subjects (6 women, 4 men). Basal serum LH (17.0 ± 0.8 IU/l) and oestradiol (women: 3.74 ± 1.84; men: 3.05 ± 0.77 pmol/l) were elevated in patients, whereas serum FSH and PRL were normal. Dopamine infusion did not modify serum FSH levels, but significantly depressed LH concentration in both hyperthyroid and healthy subjects. The LH decrement was more pronounced (P < 0.01) in patients with Graves' disease (8.4 ± 1.4 IU/l) as compared with controls (2.3 ± 0.9 IU/l). There was a positive correlation between the maximum net decrease and the basal LH concentration (r = 0.95). The per cent decrease of LH levels in the hyperthyroid patients (54 ± 4) was higher (P < 0.001) than that in the controls. The finding of enhanced sensitivity to dopamine inhibition in thyrotoxic patients suggests that their inappropriately elevated serum LH levels may result in part from a reduced dopaminergic inhibition of LH secretion.