A procedure, termed hybridization histochemistry, has been developed to locate in specially prepared whole sections of tissue those areas which contain specific mRNA populations. Three 32P-labelled recombinant DNA probes were used; one complementary to endorphin mRNA, one complementary to growth hormone mRNA and one a fragment of bacterial DNA. The specific cell populations or tissue regions binding the probe were identified by autoradiography. Hybridization histochemistry is thus similar in principle to immunohistochemical procedures. The endorphin probe consistently labelled the rat pituitary pars intermedia which is known to be particularly rich in the corresponding mRNA. Likewise the growth hormone probe specifically labelled the anterior pituitary. Control tissues were not labelled by either probe, nor did the bacterial DNA probe significantly label any tissue, providing further evidence of the specificity of the procedure. These results, which are highly reproducible, indicate that the mRNA species for endorphin and growth hormone are present in whole sections of pituitary in a physical state which leaves them accessible to cDNA probes. This initial success provides encouragement that hybridization histochemistry, with further refinement, should have wide applicability in the localization and semi-quantitative analysis of intracellular mRNA in whole frozen sections of tissue.