The first Lisp for MIT's IBM mainframe was ported to the AI lab PDP-6, running standalone. Later ITS was written for the PDP-6, and Lisp was ported to run under timesharing by 1968. This evolved into Maclisp.

MDL, first called Muddle, is mentioned in Project MAC Progress Report VIII, covering 1970-971. "The extension of LISP is known locally as "MUDDLE". It was designed and has been implemented by Carl Hewitt and Gerald Sussman of the Artificial Intelligence Laboratory and Christopher Reeve, David Cressey, Bruce Daniels, and Gregory Pfister of Dynamic Modeling".

Ideas from Muddle made it back to Maclisp. The first Lisp Machine Lisp (later ZetaLisp) was basically a reimplementation or Maclisp (as is Emacs Lisp), and Common Lisp is to a large degree a subset of Lisp Machine Lisp.