Hybridising certain AT-rich satellite complementary RNAs (cRNAs) to their homologous chromosomal DNA sequences at different temperatures of incubation results in a different dispersion of autoradiographic label throughout the karyotypes. The temperature at which most label, or cRNA-DNA hybrid formation, exists corresponds to the optimal rate temperature for the hybridisation of these same satellite cRNA-DNA hybrids as determined by RNA excess filter hybridisation. It is likely that the in situ hybridisation results can therefore be explained by the fact that there is a similar temperature-dependence on the rate of hybrid formation for both in situ and RNA excess hybridisation. This should have important implications for the designing of in situ hybridisation experiments in general.