The mutant mouse spastic (spa) develops a characteristic motor disorder about 2 weeks after birth, with symptoms resembling sublethal poisoning by the glycinergic antagonist strychnine. Correspondingly, adult homozygotic mutants (spa/spa) exhibit a severe reduction of inhibitory glycine receptors in spinal cord and brain. Here we show that the spastic mutation selectively interferes with the postnatal accumulation of the adult isoform of the glycine receptor protein, whereas perinatal expression of the neonatal receptor isoform is not detectably affected. Heterologous expression in X. laevis oocytes of poly(A)+ RNA and Northern blot analysis indicate normal levels of glycine receptor alpha 1 subunit transcripts in spinal cord of adult spastic mutants. Thus, the age-dependent manifestation of spastic symptoms after birth reflects a selective effect of the mutation on the developmental expression of the adult glycine receptor isoform.