Hypothyroid women may have various disturbances of the reproductive system. Although menstrual cycle disturbances and infertility have been reported in hypothyroidism, gonadotrophin levels have usually been found in the normal range. We have investigated whether female hypothyroid patients of reproductive age have any alteration in the pulsatile secretory pattern of gonadotrophin secretion. LH and FSH were assayed on days 2–5 of the menstrual cycle in blood samples taken every 10 min for 8 h from six hypothyroid women and six age-matched control subjects. Pulsatility was analysed using the Cluster and Detect programs. There was no significant difference in the number of peaks identified (3·7±0·8 vs 3·7±0·8 for LH, and 3·7±0·8 vs 4·2±0·5 for FSH), the mean duration of peaks (LH: 68·0±6·9 vs 72·±5·1 min; FSH: 81·9±8·1 vs 71·2±10·3 min), the area under the peaks (LH: 91·5±20·4 vs 148·2±55·1IU/l per min; FSH: 71·5±4·5 vs 62·7±15·0IU/l per min), and the incremental amplitude from baseline (LH: 2·2±0·4 vs 3·0±0·8 IU/l; FSH: 1·4±0·2 vs 2·1±0·5IU/l). However, the absolute pulse amplitude was greater in hypothyroid patients (LH: 14·5±1·4 vs 8·3±1·3 IU/l, FSH: 9·0±1·5 vs 5·8±1·2 IU/l, P=0·04), as were the integrated concentrations (LH: 6·6±0·7 vs 3·3±0·4OU/l per min, P<0·01; FSH: 4·3±0·4 vs 2·1±0·5IU/l per min, Oestradiol values were comparable in the two groups 42·7±0·4 vs 43·±9·7 pg/ml). These results indicate that in hypothyroid women there is an increased baseline level with a normal pulsatility of the gonadotrophin secretion. Similar oestrogen levels in both groups, and normal or nearnormal cycles in our patients suggest either a decreased biological potency of the gonadotrophins or a mild ovarian resistance.