A study examined the role of phonological regression in the language learning patterns of two toddlers. The children's phonological development was measured by inventories of the words produced at the beginning and end of an eight-week period, and distinctions were made between regressions due to cognitive factors and those due to non-cognitive influences. Analysis revealed four rules producing regressions in one child's data, and another three in the second child's data. The first child's regressions all involved rules operating on phonologically defined classes, and the second child's regressions operated on specific lexical items. This suggests that for these children, phonological development was not a succession of steps, each bringing the learner closer to the ambient language. Rather than being incremental, regressions led to periods in which the children's ability apparently decreased relative to earlier stages. This process is in accordance with basic tenets of cognitively-oriented models. The findings provide dramatic evidence that children can discover patterns and will implement rules even at the expense of surface phonetic accuracy. (MSE)