Several general patterns of spatial and temporal variation in fitness parameters, reflecting the nature of the random environment, are put forward and the dynamic and equilibrium behaviour of the resulting allelic frequency process is analysed. The models involve a one-locus, two-allele infinite monecious population reproducing in discrete generations by random mating in changing environments. The analyses are presented under the following headings: (1) local analysis for a general two-allele one-locus diploid model with random selection intensities; (2) a model of random selection intensities that on average is neutral (complete correlation of homozygote fitness expression); (3) the general average (neutral) random selection process with correlation of fitness expression for the homozygotes; (4) results for a general random selection process; and (5) systematic and cyclical patterns of variation in selection intensities.