In cortical pyramidal neurones, the slow afterhyperpolarising current (sI(AHP)) plays an important role in the late phase of spike frequency adaptation. sI(AHP) is a Ca 2+-dependent K+ current: in hippocampal pyramidal neurones it is activated by voltage-gated Ca 2+channels, while the role of calcium from ryanodine-sensitive intracellular stores, released by calcium-induced calcium release (CICR), is controversial (1-4, but see 5). All three types of ryanodine recptors (RyR1-3) are expressed in the hippocampal formation, with RyR3 showing a predominant expression in CA1 neurones. We investigated the specific role of RyR3-mediated CICR in the regulation of the sI(AHP) amplitude and time course and its activity-dependent potentiation (run-up) (3,5).Whole-cell patch clamp recordings of the sI(AHP) were performed on rat CA1 and CA3 and mouse CA1 pyramidal neurones in hippocampal slices, prepared in accordance with UK Home Office regulations. Ryanodine (10 µM) caused a 23.3±3.7% (n=5, p=0.04) and a 22.6±4.4% (n=6; p=0.01) reduction in the sI(AHP) amplitude in rat CA1 and CA3 neurones when applied once the current had reached stable conditions. Caffeine, known to enhance ryanodine-dependent CICR at low concentrations (0.5 mM), had a biphasic effect on the sI(AHP) in rat CA1 neurones, causing first an increase in the current amplitude (25.3±2.9%; n=22) followed by a decrease (24.5±3.6%; n=16). The caffeine-induced reduction of sI(AHP) was abolished by protein kinase A inhibitors. Application of ryanodine (10 µM) from the beginning of the recording and throughout the run-up phase of the sI(AHP) reduced the potentiation of the current measured at 15 min in rat CA1 neurones (control sI(AHP)=79.0±7.7 pA, n=5; sI(AHP) in ryanodine=51.3±7.8 pA, n=7; p=0.03). Similar results on the sI(AHP) run-up were obtained with cyclopiazonic acid (CPA; 50 µM), an endoplasmic reticulum Ca 2+-ATPase inhibitor. In CA1 neurones from mice lacking RyR3, ryanodine (10 µM) applied to the sI(AHP) after it reached stable conditions displayed a similar effect (34.5±11.7% inhibition, n=4) as in RyR3 wildtype littermates (41.1±9.8% inhibition, n=5; p=0.7). The overall potentiation of sI(AHP) was not different in RyR3 knockout (n=12) compared with wildtype mice (n=9), as judged by the final amplitude of the sI(AHP) reached at 15 min. However, the time course of sI(AHP) run-up is different in RyR3 knockout compared with their wildtype littermates. The results suggest that ryanodine-mediated CICR plays a role in the regulation of the sI(AHP), particularly in its potentiation. RyR3 does not seem to be necessary for the current generation or overall potentiation, but its absence affects the time course of the sI(AHP) run-up.