Gene dosage studies of a temperature sensitive Abruptex mutation of the Notch locus of Drosophila melanogaster revealed, firstly, that hemizygous females were lethal at all temperatures studied, while hemizygous males were subvital at 18 degrees C, semilethal at 25 degrees C, and lethal at 29 degrees C. This is due to dosage compensation of the Notch locus. Secondly, it was observed that in general an increasing dose of the wild allele decreased the expression of Abruptex at all temperatures. An exception was the Axtsl/Axtsl/ + genotype, which was more expressed than Axtsl/Axtsl at low temperatures. This suggests antimorphism as the most likely mode of action of the temperature sensitive Abruptex mutation. Thirdly, it was observed that the expression of Abruptex phenotype increased from 18 degrees C to 25 degrees C but decreased from 25 degrees C to 29 degrees C if the wild type allele was involved in the genotype. This suggests that the gene product of the temperature sensitive Abruptex allele loses its function at high temperatures. The putative antimorphic mode of action of the temperature sensitive Abruptex mutation suggests that the Notch gene product interacts with products of other neurogenic loci or with itself.