New developments in DNA sequencing techniques permit rapid progress in the determination of both repetitious and single-copy mammalian sequences. Three distinct families of highly repetitious satellite DNA's from the kangaroo rat Dipodomys ordii have been sequenced. With the MS satellite it was possible to show that the basic repeat sequence and its variants were arranged in a nonrandom order suggesting a hierarchy of repeats. The HS-alpha satellite from D. ordii was shown to resemble the guinea pig alpha satellite, a long term evolutionary persistence inconsistent with previous models. Sequences from hemoglobin mRNA were determined using hemoglobin complementary DNA as template for transcription in vitro. Seven of the largest fragments have been assigned to untranslated regions of the mRNA whereas 15 others have been tentatively located within the structural genes. From correlations with sequences from corresponding regions in the human hemoglobin mRNA's we have been able to make the first direct measurements of the rate of fixation of mutations that do not change the amino acid sequence. The minimum estimate for this rate is greater than the highest previously estimated rates of fixation of neutral mutations (calculated for fibrinopeptide A. A new technique, deoxysubstitution sequencing, which should speed determination of the complete mRNA sequences, is described.