In the present study we investigated the porphyrinogenic ability of 4 pesticides: chlordane, endosulfan, parathion and malathion, all of which are widely used in agriculture. In order to determine whether they affect the heme biosynthetic pathway we studied, in 17-day old chick embryo liver "in ovo", their effects on the amount of porphyrins and on the activity of delta-ALA synthetase (ALA-S), the first and rate limiting enzyme of this pathway. All of them induced hepatic porphyrin accumulation to a different extent compared with dimethylsulfoxide (DMSO) controls. Parathion as well as endosulfan promoted remarkable increases, chlordane raised porphyrin level in a lower degree and malathion slightly modified it. However, the accumulation observed with malathion was markedly enhanced if the period of incubation was extended to 48 h and, even more, if a second dose was injected during these 48 h. When ALA-S activity was analyzed in the chick embryos treated with parathion or endosulfan no alteration could be found in spite of producing noticeable accumulation of porphyrins. In contrast, chlordane promoted a statistically significant elevation of ALA-S as well as malathion which produced the highest induction observed. These results show that not only organochlorinated but also organophosphorous pesticides affect heme metabolism and that induction of ALA-S and porphyrin accumulation are not parallel.