1. The roles of the sympathetic nerves in regulating lipid synthesis in brown adipose tissue (BAT) were studied by measuring incorporation of 3H from 3H2O into glyceride glycerol and glyceride fatty acids in the interscapular BAT in anaesthetized rats. 2. When noradrenaline was infused intravenously at a total dose of 1-8 micrograms/100 g body weight over 30 min, 3H incorporation into glyceride glycerol increased whereas 3H incorporation into fatty acids did not change. Similar responses were found when the sympathetic nerves entering the interscapular BAT were stimulated continuously at 10 Hz. However, when electrical stimuli consisting of a much shorter train (2 s) were applied to the nerves at 3 min intervals at 10 Hz (stimulation in bursts). 3H incorporation into both glyceride glycerol and fatty acids was enhanced. Stimulation in bursts elicited more pronounced lipogenic responses than other patterns that were employed, and involved the delivery of precisely the same number of impulses over the whole period of stimulation. The lipogenic responses to nerve stimulation in bursts were increased by increasing the stimulus frequency over the range 4-40 Hz. 3. Simultaneous administration of propranolol and phenoxybenzamine had little effect on either the fatty acid or the glyceride glycerol response to nerve stimulation. In contrast, these blocking agents almost completely eliminated the responses to noradrenaline infusion. 4. Pre-treatment with guanethidine effectively abolished the lipogenic response to nerve stimulation but potentiated the response to noradrenaline infusion. 5. It is concluded that lipid synthesis in BAT is enhanced by direct electrical stimulation of the sympathetic nerves only when they are stimulated in bursts. Sympathetic activation of lipogenesis in BAT is not solely attributable to the action of noradrenaline but involves some non-adrenergic mechanism.