The plasma growth hormone response to intravenous insulin was compared with that of oral clonidine in a multicentre trial in 64 patients being investigated for short stature. Either test was judged to give a positive result if the maximum plasma growth hormone concentration was at least 20 mU/1. In 42 pairs of tests concordant results were obtained, 19 being positive and 23 negative. In 12 tests only the response to insulin was positive and 10 tests were positive only for clonidine. Clonidine caused symptoms due to hypotension in some patients but the incidence and severity of side effects varied greatly between centres. It is concluded that clonidine and insulin have similar reliability as test of growth hormone secretion, but that clonidine may be the safer.