Amitriptyline (AMT) and chlorpromazine (CPZ) (0.5 mg per animal, i.p.) were injected into rats separately for 30 days and their effects on heme metabolism in liver were examined. Significant decreases in the delta-aminolevulinate dehydratase activity were observed following the administration of both drugs (mean value of AMT-group: 6.58 U/g tissue; and CPZ-group: 7.04 U/g tissue) in comparison to that of controls (11.71 U/g tissue); however total liver heme content was not altered. When 24-h urinary excretions of delta-aminolevulinate (ALA) and porphobilinogen (PBG) were measured on the last day of the experiment, a slight (AMT-group: 38.40 micrograms/day) to distinct (CPZ-group: 59.11 micrograms/day) increase of urinary ALA was observed, while PBG excretion tended to decline only moderately under CPZ (3.52 micrograms/day), but significantly in presence of AMT (2.16 micrograms/day). Mean values obtained from control group were 32.12 micrograms/day for ALA and 4.25 micrograms/day for PBG.