Properties of low Mg2+ induced epileptiform activity were studied in isolated rat hippocampal slices or in combined slices containing the entorhinal cortex and hippocampus. Slices were prepared from rats which were 1, 2, 3 or more weeks of age. Field potentials and often also changes in [K+]0, [Ca2+]0 and [Mg2+]0 were recorded with appropriate ion selective microelectrodes. In isolated hippocampal and entorhinal cortex/hippocampal combined slices the latency to onset of epileptiform activity upon lowering of extracellular Mg2+ was shortest in the youngest age group and approached adult levels at about the fourth postnatal week. Washout kinetics of Mg2+ were fastest in slices from 1-week-old rats. The onset of low Mg2+ induced epileptiform activity occurred at higher Mg2+ levels in slices from young compared with those from adult animals. In isolated hippocampal slices the epileptiform discharges varied in appearance during development. Short discharges lasting for 40 to 80 ms were observed in hippocampal slices prepared from 1-week-old and adult animals. Seizure-like events (SLE's) characterized by slow negative potential shifts and characteristic elevations in [K +]0 and decreases in [Ca2+]0 lasting for up to 30 s were observed in a proportion of hippocampal slices prepared after the first, second and third postnatal week. In slices from week 2 and 3 seizure-like events often progressed into spreading depressions (SD's). In entorhinal cortex/hippocampal combined slices seizure-like events were observed in all age groups. The seizure-like events spread readily into dentate gyrus (DG), area CA3 and CA1 after week 1.(ABSTRACT TRUNCATED AT 250 WORDS)