Pre-embedding and post-embedding in situ hybridization techniques were compared for the localization of RNAs in the nucleus. 28S rRNA and transcripts of the epidermal growth factor receptor (EGF-receptor) were localized with both hybridization methods. Pre-embedding hybridizations were performed on cells permeabilized with Triton X-100, whereas post-embedding hybridizations were carried out on Lowicryl K4M sections. From these studies it was concluded that, for labelling of 28S rRNA, the post-embedding in situ hybridization is preferred, whereas EGF-receptor transcripts were successfully detected only after pre-embedding in situ hybridization. Furthermore, the detection of the hybrids with ultra-small gold particles was compared to the detection with 6 nm gold particles in both pre- and post-embedding in situ hybridization studies. From our results it is concluded that the use of ultra-small gold particles results in higher label efficiency. Therefore, ultra-small gold particles are preferable to 6 nm gold particles for the detection of hybrids in high-resolution in situ hybridization experiments.