The hypoglossal nucleus in 129 REJ normal mouse strains was investigated using two neuroanatomical markers, namely the cobalt chloride (CoCl2) and the horseradish peroxidase (HRP) techniques. CoCl2 was introduced through the cut end of the hypoglossal nerve. In one set of experiments HRP was injected into the hypoglossal nerve, while in the other it was injected into the tongue musculature. Results show that with these techniques the hypoglossal neurons are conspicuously stained and can be easily located among series of brainstem sections. The mean number +/- SD of neurons in the hypoglossal nucleus was 1,417 +/- 37, 846 +/- 28 and 1,272 +/- 42 using CoCl2 and HRP injected into the tongue musculature or the hypoglossal nerve, respectively. The estimated length of the nucleus was 0.92 mm with the CoCl2 technique.