1. The membrane currents elicited by L-glutamate and glycine applications in morphologically different neurone types were investigated in isolated spinal cord cells from the lumbar enlargement of 6 to 11-day-old chick embryos. The whole-cell patch-clamp technique and concentration clamp methods have been used. Isolated spinal cord neurones of four stages were investigated: 6, 7.5, 9 and 11 days of incubation (29th, 32nd, 35th and 37th stages of development, respectively). 2. The L-glutamate-activated conductance consisted of desensitizing and non-desensitizing components. The Hill coefficient for the first component was 1, and for the second was 2. The number of cells responding to L-glutamate application with only a desensitizing component decreased from 53.4% on the 6th day of incubation to 6.7% on the 11th day, whereas the number of cells responding with bicomponent responses increased during the same period from 13.3 to 87%. 3. From the 6th and 11th day of chick embryo development the characteristics of the desensitizing component of L-glutamate-activated conductance remained constant (half-maximal dose, (ED50 = 2.6 +/- 0.3 mM) whereas the ED50 for the non-desensitizing component decreased 10 times. 4. It was found that the density of the desensitizing L-glutamate-activated ionic current increased during morphological and age-dependent differentiation of spinal cord cells. 5. Of the investigated cells 88.7% were sensitive to glycine application. The smallest percentage of neurones responding to glycine application was observed at 7.5 days of incubation. Glycine-activated conductance did not change at the investigated stages (ED50, 71 +/- 2 microM; Hill coefficient, 2). 6. A significant decrease in the glycine-activated current density was observed on the 9th day of incubation in multipolar neurones with three neurites, against a tendency to an increase of glycine-activated ionic currents during morphological and age-dependent differentiation of cells. 7. Of the investigated cells 78% were sensitive to both glycine and L-glutamate application. A negative correlation between glycine- and L-glutamate-activated current densities (correlation coefficient, -0.71) was revealed by means of statistical analysis. 8. We conclude that these changes in chemosensitivity of the spinal cord neurones may underlie the increase in locomotor activity of the chick embryo observed on the 9th day of incubation.