We had earlier found that granule neurons in cultures of small explants of neonatal rat cerebellar cortex could be placed in two groups: cells in one group showed spontaneous synaptic activity and also had a large response to applications of 1 microM gamma-aminobutyric acid (GABA) while cells in the other lacked spontaneous activity and also showed much lower sensitivity to GABA [25]. For convenience, the more responsive cells will be termed A-type neurons, while the less responsive cells will be termed B-type. We have undertaken a comparison of the responses mediated by activation of GABA A receptors for the two types of neurons. A-type neurons have a larger maximal response to GABA (about 10 times that for B-type neurons), suggesting that they express more functional GABA(A) receptors. The concentration of GABA producing half-maximal activation of A-type neurons is somewhat less (12 mu M) than that for B-type neurons (41 microM), while the Hill coefficients are similar. Responses of both types of cell desensitize to prolonged applications of GABA. At a given concentration of GABA the responses of A-type neurons desensitize more rapidly than the responses of B-type neurons, indicating that the physiological properties of the receptors differ. Responses of A-type neurons are also potentiated to a significantly lesser extent by either chlordiazepoxide or alphaxalone than are the responses of B-type neurons, indicating that the pharmacological properties of the receptors differ. These data indicate that A-type and B-type granule neurons in our cultures express GABA(A) receptors which differ in number, physiological properties and pharmacological responsiveness. We have also confirmed the observation that almost all A-type neurons also show spontaneous synaptic currents, while almost no B-type neurons do.