1. A possible relation between the family of inwardly rectifying K+ channels and the Shaker superfamily of K+ channels was investigated using a deletion mutant (DelS1-S4) of a delayed rectifier Kv1.1 (RCK1) K+ channel. 2. The mutant DelS1-S4 was made by eliminating the sequence coding for transmembrane domains S1 to S4 of the Kv1.1 K+ channel, and re-ligating the sequence coding for the cytoplasmic amino terminus to transmembrane domain S5. Microelectrode voltage-clamp and patch-clamp experiments were performed on Xenopus laevis oocytes after injection of in vitro transcribed mRNA coding for mutant and wild-type channels. 3. The lack of transmembrane domains S1 to S4 converts a depolarization-activated wild-type Kv1.1 K+ channel with outward rectification into a hyperpolarization-activated channel with inward rectification. Although the pore region of the deletion mutant is identical to the wild-type channel, the mutant channel is a non-selective cation channel and is characterized by an altered pharmacology profile.