We cloned 10 full length variants of PIIH, the gene for I_h from the spiny lobster, Panulirus interruptus, using reverse transcription-PCR (RT-PCR) and rapid amplification of cDNA ends (RACE). This gene shows a significant amount of alternative splicing in the S3-S4 and S4-S5 linkers, in the P-loop and the entire S6 transmembrane domain, in the cyclic nucleotide binding domain (CNBD), and near the 3’ end of the gene. Functional expression of 7 splice variants in Xenopus oocytes generated slowly activating hyperpolarization-activated inward currents, which were blocked by the I_h channel blockers CsCl and ZD7288. The different splice variants had markedly varying activation kinetics and voltage dependence of activation. Bath application of 8-Br-cAMP shifted the V_1/2 to more positive potentials and accelerated the activation kinetics in an isoform-specific manner. Two variants containing a segment with an ER-retention motif in the S4-S5 loop did not produce currents in oocytes. Overexpression of one splice variant, PIIH AB_S-I, in Pyloric Dilator (PD) neurons in the lobster stomatogastric ganglion produced an average 3-fold increase in I_h without evoking a compensatory increase in I_A. The voltage for half-maximal activation of I_h in PIIH AB_S-I-expressing PDs was shifted in the depolarizing direction by 9 mV, whereas the slope factor decreased by 3.8 mV. Moreover, its activation kinetics were significantly faster than in control PDs. PIIH AB_S-I overexpression enhanced PD neuron rhythmic firing in an amplitude-dependent manner above a minimal threshold 2- to 3-fold increase in amplitude.