This paper describes ZEN, a directive-based language for the specification of arbitrarily complex program executions by varying the problem, system, or machine parameters for parallel and distributed applications. ZEN introduces directives to substitute strings and to insert assignment statements inside arbitrary files, such as program, input, script, or make-files. The programmer thus can invoke experiments for arbitrary value ranges of any problem parameter, including program variables, file names, compiler options, target machines, machine sizes, scheduling strategies, data distributions, etc. The number of experiments can be controlled through ZEN constraint directives. Finally, the programmer may request a large set of performance metrics to be computed for any code region of interest. The scope of ZEN directives can be restricted to arbitrary file or code regions. We implemented a prototype tool for automatic experiment management that is based on ZEN. We report results for the performance analysis of an ocean simulation application and for the parameter study of a computational finance code.