In previous studies primary sensory neurons of adult rats have been counted in lumbar dorsal root ganglia. However, different counting methods have given very different results and at the cervical level, recent data are scarce. In the present study, the number of neurons in C4, C5 and C6 adult rat ganglia was determined using two previously calibrated techniques. The stereological tool was preferred because it directly identifies neurons instead of nucleoli and is more efficient. The C4, C5 and C6 dorsal root ganglia were found to contain 7508+/-299, 6825+/-950 and 6858+/-923 neurons, respectively, and statistical analysis indicated that there was no significant difference between the three levels. There was, however, a great interindividual variation, which was also found at other levels of the spinal cord. The mean diameter of neurons in the C4, C5 and C6 dorsal root ganglia was determined and was 17.52, 20.16 and 20.68 microm, respectively. It is important to know more about the organization of the sensory systems in the normal rat. Once established, the number of neurons in these dorsal root ganglia could be compared with different pathological situations or experimental treatments such as developmental conditions, nerve section or ganglion transplantation.