The Drosophila frizzled (fz) gene is required for the development of normal tissue polarity in the epidermis. Genetic epistasis experiments argue that fz is at the top of a regulatory hierarchy that controls the subcellular site for prehair initiation within the cells of the pupal wing (Wong and Adler, 1993; J. Cell Biol. 123, 209-221). Genetic mosaic experiments indicate that fz has both cell autonomous and cell non-autonomous functions that are separately mutable (Vinson and Adler, 1987; Nature 329, 549-551). Two species of fz mRNA have been identified, raising the question as to whether the two functions are provided by a single protein or by two separate protein species. We generated transgenic flies that express each of these mRNAs under the control of an hsp70 promoter. Only one of the transgenes (hsfzI) showed any fz activity. At 29 degrees C, the hsfzI transgene provided almost complete rescue of a null fz mutation, indicating that the protein encoded by this cDNA can fulfill both fz functions. Overexpression of the hsfzI transgene resulted in two distinct tissue polarity phenotypes depending on the time of heat shock.