Administering ${delta}-aminolevulinic$ acid (ALA) to isolated pea (Pisum sativum L.) chloroplasts resulted in an increase of heme synthesis in the heme branch of the tetrapyrrole pathway. At 0.1 mM ALA, in the presence of 1 mM $FeSO_4$ heme synthesis was stimulated up to 7 fold of that in the absence of $FeSO_4$. N-Methylmesoporphyrin IX (NMMP), a powerful inhibitor of ferrochelatase, inhibited heme synthesis by 95% at one micromolar concentration. The addition of A TP to the chloroplasts caused not only heme synthesis, but Mg-protoporphyrin IX synthesis in the chlorophyll branch of the tetrapyrrole pathway. In the presence of NMMP, however, inhibition of Mg-protoporphyrin IX synthesis was not observed whereas heme synthesis was inhibited completely.