This study investigated the effect of two external conditions on the learning of principles by children. The external conditions were instruction on the major concepts related in the principle and the number of teaching examples and teaching nonexamples of the principle presented with a statement of the principle. Subjects were 159 fifth-grade students of average ability and socioeconomic status. The principles were presented in self-instructional lesson booklets, where the instructional variables of interest were systematically varied. Variations in the type of lessons read by the subjects constituted the experimental conditions. Mastery of the principles was determined by tests designed to assess subjects' knowledge of, and ability to apply, the principles. Results indicated that a rationally selected set of examples and nonexamples was more facilitative than one example, although it was concluded that, for easy principles, providing one example was equally effective as a rationally selected set when instruction on concepts related in the principle was also provided. Main effects for instruction on concepts related in the principles were not found to be significant. The conditions associated with this unanticipated result were discussed. Extensive appendices are included. (Author/PC)