Código HTML:
<form name="f1"> <script language="javascript" type="text/javascript"> function CambiaEstado(valor){ if (valor == true){ document.f1.espec.disabled = "disabled"; }else{ document.f1.espec.disabled = false; } }</script> <select name="espec" style='width:162px;' class="cajatexto"> <option selected="selected"></option> <option>Canino</option> <option>Felino</option> <option>Conejo</option> <option>Exoticos</option> <option>Otros</option> </select> <br/><br/> <input type="checkbox" name="chk" checked="checked" onclick="CambiaEstado(this.checked);" /> </form>