The glucagon response to insulin-induced hypoglycaemia was tested in rats that had been pinealectomised (Px), pinealectomised and fitted with melatonin implants (Px + MLT), or sham-operated (SO). The glucagon response to hypoglycaemia observed in SO rats (216 ± 27 pg ml-1 at baseline versus 397 ± 35 pg ml-1 at the hypoglycaemic peak, mean ± S.D.) was stronger than that in Px rats (180 ± 37 pg ml-1 and 229 ± 21 pg ml-1, respectively) and weaker than that in Px + MLT rats (256 ± 19 pg ml-1 and 516 ± 11 pg ml-1, respectively). Our data indicate that the capacity to release glucagon during insulin-induced hypoglycaemia is altered in pinealectomised rats. Experimental Physiology (2001) 86.5, 617-620.