diff --git a/collects/scribblings/reference/debugging.scrbl b/collects/scribblings/reference/debugging.scrbl
index 87e87bc4dc..2513ee2e79 100644
--- a/collects/scribblings/reference/debugging.scrbl
+++ b/collects/scribblings/reference/debugging.scrbl
@@ -5,9 +5,9 @@
 
 Racket's built-in debugging support is limited to context (i.e.,
 ``stack trace'') information that is printed with an exception. In
-some cases, disabling the JIT compiler can affect context
+some cases, disabling the @tech{JIT} compiler can affect context
 information. The @racketmodname[errortrace] library supports more
-consistent (independent of the JIT compiler) and precise context
+consistent (independent of the @tech{JIT} compiler) and precise context
 information.  The @racketmodname[racket/trace] library provides simple
 tracing support. Finally, the @seclink[#:doc '(lib
 "scribblings/drracket/drracket.scrbl") "top"]{DrRacket} programming environment
diff --git a/collects/scribblings/reference/eval.scrbl b/collects/scribblings/reference/eval.scrbl
index 474401f43b..41de77867d 100644
--- a/collects/scribblings/reference/eval.scrbl
+++ b/collects/scribblings/reference/eval.scrbl
@@ -460,11 +460,17 @@ which allows such optimizations.}
 
 @defboolparam[eval-jit-enabled on?]{
 
+@guidealso["JIT"]
+
 A parameter that determines whether the native-code just-in-time
-compiler (JIT) is enabled for code (compiled or not) that is passed to
-the default evaluation handler.  The default is @racket[#t], unless
-the JIT is disabled through the @Flag{j}/@DFlag{no-jit} command-line
-flag to stand-alone Racket (or GRacket), or through the
+compiler (@deftech{JIT}) is enabled for code (compiled or not) that is passed to
+the default evaluation handler. A true parameter value is effective
+only on platforms for which the JIT is supported.
+
+The default is @racket[#t], unless the JIT is not supported by the
+current platform, unless it is disabled through the
+@Flag{j}/@DFlag{no-jit} command-line flag to stand-alone Racket (or
+GRacket), and unless it is disabled through the
 @as-index{@envvar{PLTNOMZJIT}} environment variable (set to any
 value).}
 
diff --git a/collects/scribblings/reference/runtime.scrbl b/collects/scribblings/reference/runtime.scrbl
index 330279c83a..4e65b7daf6 100644
--- a/collects/scribblings/reference/runtime.scrbl
+++ b/collects/scribblings/reference/runtime.scrbl
@@ -125,7 +125,7 @@ treated as flags for the system).}
 
 A parameter that provides a hint about how much space to reserve for a
 newly created thread's local variables. The actual space used by a
-computation is affected by just-in-time (JIT) compilation, but it is
+computation is affected by @tech{JIT} compilation, but it is
 otherwise platform-independent.}
 
 
diff --git a/src/racket/src/eval.c b/src/racket/src/eval.c
index a421bd8ef0..c0735edfd5 100644
--- a/src/racket/src/eval.c
+++ b/src/racket/src/eval.c
@@ -158,8 +158,14 @@
 # include "future.h"
 #endif
 
+#ifdef MZ_USE_JIT
+# define INIT_JIT_ON 1
+#else
+# define INIT_JIT_ON 0
+#endif
+
 /* globals */
-SHARED_OK int scheme_startup_use_jit = 1;
+SHARED_OK int scheme_startup_use_jit = INIT_JIT_ON;
 void scheme_set_startup_use_jit(int v) { scheme_startup_use_jit =  v; }
 
 /* THREAD LOCAL SHARED */