Diverse  higher  plant  species  synthesize  low  molecular  weight  (LMW)  heat  shock  proteins  (HSPs)  which  localize  to  chloroplasts.  These  proteins  are homologous  to  LMW  HSPs  found  in  the  cytoplasm of  all  eukaryotes,  a class  of  HSPs  whose  molecular mode  of  action  is  not  understood.  To  obtain  basic  information  concerning  the  role  of  chloroplast  HSPs,  we  examined  the  accumulation,  stability,  tissue   specificity,  and  intra-chloroplast  localization  of  HSP21,  the  major  LMW  chloroplast  HSP  in  pea.  Intact  pea  plants  were  subjected  to  heat  stress  conditions  which would  be  encountered  in  the  natural  environment  and  HSP21  mRNA  and  protein  levels  were  measured  in  leaves  and  roots.  HSP21  was  not  detected  in  leaves  or  roots  before stress,  but  the mature,  21-kD  protein  accumulated  in  direct  proportion  to  temperature  and  HSP21  mRNA  levels  in  both  tissues.  All  of  the  HSP21  in  leaves  was  localized  to  chloroplasts;  there  was  no  evidence  for  its  transport  into  other  organelles.  In  chloroplast  fractionation   experiments,  >80%  of  HSP21  was  recovered  in  the soluble chloroplast protein fraction. The half-life of HSP21 at control temperatures was 52 + 12 h, suggesting the protein's function is critical during recovery as well as during stress. We hypothesize that HSP21 functions in a catalytic fashion in both pho- tosynthetic and nonphotosynthetic plastids.