The monopyrrole, 3-ethyl-5-hydroxy-4,5-dimethyl-delta 3-pyrroline-2-one (HPL), was given by intraperitoneal injection to rats (0.65 mmol/kg) and to mice (0.98 and 1.95 mmol/kg). Behaviour of the rats was assessed for 1.5 hr after injection by monitoring their spontaneous activity, while behaviour of the mice was examined by ethological procedures during dyadic encounters over a 5 min. period at 50-60 min. following injection. Activity of the rats, once habituated to the cage, was reduced by HPL. No other abnormalities were discernible. In mice, HPL at 1.95 mmol/kg increased the frequency of "head twitch" and evoked a smaller increase of "backward locomotion". There were no changes in overall agonistic, social or non-social behaviour in the HPL treated mice. Plasma concentrations of HPL in mice treated with the highest dose level amounted to 0.3 +/- 0.1 mmol/l, many-fold above its plasma concentrations during acute porphyric attacks in man. It thus appears that in rodents HPL has slight behavioural activity, but that it is unlikely to play any significant role in the psychiatric disturbances of acute porphyria.